drbd_req.c

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/*
   drbd_req.c

   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.

   Copyright (C) 2001-2008, LINBIT Information Technologies GmbH.
   Copyright (C) 1999-2008, Philipp Reisner <[email protected]>.
   Copyright (C) 2002-2008, Lars Ellenberg <[email protected]>.

   drbd is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   drbd is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with drbd; see the file COPYING.  If not, write to
   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

 */

#include <linux/module.h>

#include <linux/slab.h>
#include <linux/drbd.h>
#include "drbd_int.h"
#include "drbd_req.h"


/* Update disk stats at start of I/O request */
static void _drbd_start_io_acct(struct drbd_conf *mdev, struct drbd_request *req, struct bio *bio)
{
    const int rw = bio_data_dir(bio);
    int cpu;
    cpu = part_stat_lock();
    part_round_stats(cpu, &mdev->vdisk->part0);
    part_stat_inc(cpu, &mdev->vdisk->part0, ios[rw]);
    part_stat_add(cpu, &mdev->vdisk->part0, sectors[rw], bio_sectors(bio));
    part_inc_in_flight(&mdev->vdisk->part0, rw);
    part_stat_unlock();
}

/* Update disk stats when completing request upwards */
static void _drbd_end_io_acct(struct drbd_conf *mdev, struct drbd_request *req)
{
    int rw = bio_data_dir(req->master_bio);
    unsigned long duration = jiffies - req->start_time;
    int cpu;
    cpu = part_stat_lock();
    part_stat_add(cpu, &mdev->vdisk->part0, ticks[rw], duration);
    part_round_stats(cpu, &mdev->vdisk->part0);
    part_dec_in_flight(&mdev->vdisk->part0, rw);
    part_stat_unlock();
}

static void _req_is_done(struct drbd_conf *mdev, struct drbd_request *req, const int rw)
{
    const unsigned long s = req->rq_state;

    /* remove it from the transfer log.
     * well, only if it had been there in the first
     * place... if it had not (local only or conflicting
     * and never sent), it should still be "empty" as
     * initialized in drbd_req_new(), so we can list_del() it
     * here unconditionally */
    list_del(&req->tl_requests);

    /* if it was a write, we may have to set the corresponding
     * bit(s) out-of-sync first. If it had a local part, we need to
     * release the reference to the activity log. */
    if (rw == WRITE) {
        /* Set out-of-sync unless both OK flags are set
         * (local only or remote failed).
         * Other places where we set out-of-sync:
         * READ with local io-error */
        if (!(s & RQ_NET_OK) || !(s & RQ_LOCAL_OK))
            drbd_set_out_of_sync(mdev, req->sector, req->size);

        if ((s & RQ_NET_OK) && (s & RQ_LOCAL_OK) && (s & RQ_NET_SIS))
            drbd_set_in_sync(mdev, req->sector, req->size);

        /* one might be tempted to move the drbd_al_complete_io
         * to the local io completion callback drbd_endio_pri.
         * but, if this was a mirror write, we may only
         * drbd_al_complete_io after this is RQ_NET_DONE,
         * otherwise the extent could be dropped from the al
         * before it has actually been written on the peer.
         * if we crash before our peer knows about the request,
         * but after the extent has been dropped from the al,
         * we would forget to resync the corresponding extent.
         */
        if (s & RQ_LOCAL_MASK) {
            if (get_ldev_if_state(mdev, D_FAILED)) {
                if (s & RQ_IN_ACT_LOG)
                    drbd_al_complete_io(mdev, req->sector);
                put_ldev(mdev);
            } else if (__ratelimit(&drbd_ratelimit_state)) {
                dev_warn(DEV, "Should have called drbd_al_complete_io(, %llu), "
                     "but my Disk seems to have failed :(\n",
                     (unsigned long long) req->sector);
            }
        }
    }

    drbd_req_free(req);
}

static void queue_barrier(struct drbd_conf *mdev)
{
    struct drbd_tl_epoch *b;

    /* We are within the req_lock. Once we queued the barrier for sending,
     * we set the CREATE_BARRIER bit. It is cleared as soon as a new
     * barrier/epoch object is added. This is the only place this bit is
     * set. It indicates that the barrier for this epoch is already queued,
     * and no new epoch has been created yet. */
    if (test_bit(CREATE_BARRIER, &mdev->flags))
        return;

    b = mdev->newest_tle;
    b->w.cb = w_send_barrier;
    /* inc_ap_pending done here, so we won't
     * get imbalanced on connection loss.
     * dec_ap_pending will be done in got_BarrierAck
     * or (on connection loss) in tl_clear.  */
    inc_ap_pending(mdev);
    drbd_queue_work(&mdev->data.work, &b->w);
    set_bit(CREATE_BARRIER, &mdev->flags);
}

static void _about_to_complete_local_write(struct drbd_conf *mdev,
    struct drbd_request *req)
{
    const unsigned long s = req->rq_state;
    struct drbd_request *i;
    struct drbd_epoch_entry *e;
    struct hlist_node *n;
    struct hlist_head *slot;

    /* Before we can signal completion to the upper layers,
     * we may need to close the current epoch.
     * We can skip this, if this request has not even been sent, because we
     * did not have a fully established connection yet/anymore, during
     * bitmap exchange, or while we are C_AHEAD due to congestion policy.
     */
    if (mdev->state.conn >= C_CONNECTED &&
        (s & RQ_NET_SENT) != 0 &&
        req->epoch == mdev->newest_tle->br_number)
        queue_barrier(mdev);

    /* we need to do the conflict detection stuff,
     * if we have the ee_hash (two_primaries) and
     * this has been on the network */
    if ((s & RQ_NET_DONE) && mdev->ee_hash != NULL) {
        const sector_t sector = req->sector;
        const int size = req->size;

        /* ASSERT:
         * there must be no conflicting requests, since
         * they must have been failed on the spot */
#define OVERLAPS overlaps(sector, size, i->sector, i->size)
        slot = tl_hash_slot(mdev, sector);
        hlist_for_each_entry(i, n, slot, collision) {
            if (OVERLAPS) {
                dev_alert(DEV, "LOGIC BUG: completed: %p %llus +%u; "
                      "other: %p %llus +%u\n",
                      req, (unsigned long long)sector, size,
                      i, (unsigned long long)i->sector, i->size);
            }
        }

        /* maybe "wake" those conflicting epoch entries
         * that wait for this request to finish.
         *
         * currently, there can be only _one_ such ee
         * (well, or some more, which would be pending
         * P_DISCARD_ACK not yet sent by the asender...),
         * since we block the receiver thread upon the
         * first conflict detection, which will wait on
         * misc_wait.  maybe we want to assert that?
         *
         * anyways, if we found one,
         * we just have to do a wake_up.  */
#undef OVERLAPS
#define OVERLAPS overlaps(sector, size, e->sector, e->size)
        slot = ee_hash_slot(mdev, req->sector);
        hlist_for_each_entry(e, n, slot, collision) {
            if (OVERLAPS) {
                wake_up(&mdev->misc_wait);
                break;
            }
        }
    }
#undef OVERLAPS
}

void complete_master_bio(struct drbd_conf *mdev,
        struct bio_and_error *m)
{
    bio_endio(m->bio, m->error);
    dec_ap_bio(mdev);
}

/* Helper for __req_mod().
 * Set m->bio to the master bio, if it is fit to be completed,
 * or leave it alone (it is initialized to NULL in __req_mod),
 * if it has already been completed, or cannot be completed yet.
 * If m->bio is set, the error status to be returned is placed in m->error.
 */
void _req_may_be_done(struct drbd_request *req, struct bio_and_error *m)
{
    const unsigned long s = req->rq_state;
    struct drbd_conf *mdev = req->mdev;
    int rw = req->rq_state & RQ_WRITE ? WRITE : READ;

    /* we must not complete the master bio, while it is
     *  still being processed by _drbd_send_zc_bio (drbd_send_dblock)
     *  not yet acknowledged by the peer
     *  not yet completed by the local io subsystem
     * these flags may get cleared in any order by
     *  the worker,
     *  the receiver,
     *  the bio_endio completion callbacks.
     */
    if (s & RQ_NET_QUEUED)
        return;
    if (s & RQ_NET_PENDING)
        return;
    if (s & RQ_LOCAL_PENDING && !(s & RQ_LOCAL_ABORTED))
        return;

    if (req->master_bio) {
        /* this is data_received (remote read)
         * or protocol C P_WRITE_ACK
         * or protocol B P_RECV_ACK
         * or protocol A "handed_over_to_network" (SendAck)
         * or canceled or failed,
         * or killed from the transfer log due to connection loss.
         */

        /*
         * figure out whether to report success or failure.
         *
         * report success when at least one of the operations succeeded.
         * or, to put the other way,
         * only report failure, when both operations failed.
         *
         * what to do about the failures is handled elsewhere.
         * what we need to do here is just: complete the master_bio.
         *
         * local completion error, if any, has been stored as ERR_PTR
         * in private_bio within drbd_endio_pri.
         */
        int ok = (s & RQ_LOCAL_OK) || (s & RQ_NET_OK);
        int error = PTR_ERR(req->private_bio);

        /* remove the request from the conflict detection
         * respective block_id verification hash */
        if (!hlist_unhashed(&req->collision))
            hlist_del(&req->collision);
        else
            D_ASSERT((s & (RQ_NET_MASK & ~RQ_NET_DONE)) == 0);

        /* for writes we need to do some extra housekeeping */
        if (rw == WRITE)
            _about_to_complete_local_write(mdev, req);

        /* Update disk stats */
        _drbd_end_io_acct(mdev, req);

        m->error = ok ? 0 : (error ?: -EIO);
        m->bio = req->master_bio;
        req->master_bio = NULL;
    }

    if (s & RQ_LOCAL_PENDING)
        return;

    if ((s & RQ_NET_MASK) == 0 || (s & RQ_NET_DONE)) {
        /* this is disconnected (local only) operation,
         * or protocol C P_WRITE_ACK,
         * or protocol A or B P_BARRIER_ACK,
         * or killed from the transfer log due to connection loss. */
        _req_is_done(mdev, req, rw);
    }
    /* else: network part and not DONE yet. that is
     * protocol A or B, barrier ack still pending... */
}

static void _req_may_be_done_not_susp(struct drbd_request *req, struct bio_and_error *m)
{
    struct drbd_conf *mdev = req->mdev;

    if (!is_susp(mdev->state))
        _req_may_be_done(req, m);
}

/*
 * checks whether there was an overlapping request
 * or ee already registered.
 *
 * if so, return 1, in which case this request is completed on the spot,
 * without ever being submitted or send.
 *
 * return 0 if it is ok to submit this request.
 *
 * NOTE:
 * paranoia: assume something above us is broken, and issues different write
 * requests for the same block simultaneously...
 *
 * To ensure these won't be reordered differently on both nodes, resulting in
 * diverging data sets, we discard the later one(s). Not that this is supposed
 * to happen, but this is the rationale why we also have to check for
 * conflicting requests with local origin, and why we have to do so regardless
 * of whether we allowed multiple primaries.
 *
 * BTW, in case we only have one primary, the ee_hash is empty anyways, and the
 * second hlist_for_each_entry becomes a noop. This is even simpler than to
 * grab a reference on the net_conf, and check for the two_primaries flag...
 */
static int _req_conflicts(struct drbd_request *req)
{
    struct drbd_conf *mdev = req->mdev;
    const sector_t sector = req->sector;
    const int size = req->size;
    struct drbd_request *i;
    struct drbd_epoch_entry *e;
    struct hlist_node *n;
    struct hlist_head *slot;

    D_ASSERT(hlist_unhashed(&req->collision));

    if (!get_net_conf(mdev))
        return 0;

    /* BUG_ON */
    ERR_IF (mdev->tl_hash_s == 0)
        goto out_no_conflict;
    BUG_ON(mdev->tl_hash == NULL);

#define OVERLAPS overlaps(i->sector, i->size, sector, size)
    slot = tl_hash_slot(mdev, sector);
    hlist_for_each_entry(i, n, slot, collision) {
        if (OVERLAPS) {
            dev_alert(DEV, "%s[%u] Concurrent local write detected! "
                  "[DISCARD L] new: %llus +%u; "
                  "pending: %llus +%u\n",
                  current->comm, current->pid,
                  (unsigned long long)sector, size,
                  (unsigned long long)i->sector, i->size);
            goto out_conflict;
        }
    }

    if (mdev->ee_hash_s) {
        /* now, check for overlapping requests with remote origin */
        BUG_ON(mdev->ee_hash == NULL);
#undef OVERLAPS
#define OVERLAPS overlaps(e->sector, e->size, sector, size)
        slot = ee_hash_slot(mdev, sector);
        hlist_for_each_entry(e, n, slot, collision) {
            if (OVERLAPS) {
                dev_alert(DEV, "%s[%u] Concurrent remote write detected!"
                      " [DISCARD L] new: %llus +%u; "
                      "pending: %llus +%u\n",
                      current->comm, current->pid,
                      (unsigned long long)sector, size,
                      (unsigned long long)e->sector, e->size);
                goto out_conflict;
            }
        }
    }
#undef OVERLAPS

out_no_conflict:
    /* this is like it should be, and what we expected.
     * our users do behave after all... */
    put_net_conf(mdev);
    return 0;

out_conflict:
    put_net_conf(mdev);
    return 1;
}

/* obviously this could be coded as many single functions
 * instead of one huge switch,
 * or by putting the code directly in the respective locations
 * (as it has been before).
 *
 * but having it this way
 *  enforces that it is all in this one place, where it is easier to audit,
 *  it makes it obvious that whatever "event" "happens" to a request should
 *  happen "atomically" within the req_lock,
 *  and it enforces that we have to think in a very structured manner
 *  about the "events" that may happen to a request during its life time ...
 */
int __req_mod(struct drbd_request *req, enum drbd_req_event what,
        struct bio_and_error *m)
{
    struct drbd_conf *mdev = req->mdev;
    int rv = 0;
    m->bio = NULL;

    switch (what) {
    default:
        dev_err(DEV, "LOGIC BUG in %s:%u\n", __FILE__ , __LINE__);
        break;

    /* does not happen...
     * initialization done in drbd_req_new
    case created:
        break;
        */

    case to_be_send: /* via network */
        /* reached via drbd_make_request_common
         * and from w_read_retry_remote */
        D_ASSERT(!(req->rq_state & RQ_NET_MASK));
        req->rq_state |= RQ_NET_PENDING;
        inc_ap_pending(mdev);
        break;

    case to_be_submitted: /* locally */
        /* reached via drbd_make_request_common */
        D_ASSERT(!(req->rq_state & RQ_LOCAL_MASK));
        req->rq_state |= RQ_LOCAL_PENDING;
        break;

    case completed_ok:
        if (req->rq_state & RQ_WRITE)
            mdev->writ_cnt += req->size>>9;
        else
            mdev->read_cnt += req->size>>9;

        req->rq_state |= (RQ_LOCAL_COMPLETED|RQ_LOCAL_OK);
        req->rq_state &= ~RQ_LOCAL_PENDING;

        _req_may_be_done_not_susp(req, m);
        break;

    case abort_disk_io:
        req->rq_state |= RQ_LOCAL_ABORTED;
        if (req->rq_state & RQ_WRITE)
            _req_may_be_done_not_susp(req, m);
        else
            goto goto_queue_for_net_read;
        break;

    case write_completed_with_error:
        req->rq_state |= RQ_LOCAL_COMPLETED;
        req->rq_state &= ~RQ_LOCAL_PENDING;

        __drbd_chk_io_error(mdev, DRBD_IO_ERROR);
        _req_may_be_done_not_susp(req, m);
        break;

    case read_ahead_completed_with_error:
        /* it is legal to fail READA */
        req->rq_state |= RQ_LOCAL_COMPLETED;
        req->rq_state &= ~RQ_LOCAL_PENDING;
        _req_may_be_done_not_susp(req, m);
        break;

    case read_completed_with_error:
        drbd_set_out_of_sync(mdev, req->sector, req->size);

        req->rq_state |= RQ_LOCAL_COMPLETED;
        req->rq_state &= ~RQ_LOCAL_PENDING;

        if (req->rq_state & RQ_LOCAL_ABORTED) {
            _req_may_be_done(req, m);
            break;
        }

        __drbd_chk_io_error(mdev, DRBD_IO_ERROR);

    goto_queue_for_net_read:

        D_ASSERT(!(req->rq_state & RQ_NET_MASK));

        /* no point in retrying if there is no good remote data,
         * or we have no connection. */
        if (mdev->state.pdsk != D_UP_TO_DATE) {
            _req_may_be_done_not_susp(req, m);
            break;
        }

        /* _req_mod(req,to_be_send); oops, recursion... */
        req->rq_state |= RQ_NET_PENDING;
        inc_ap_pending(mdev);
        /* fall through: _req_mod(req,queue_for_net_read); */

    case queue_for_net_read:
        /* READ or READA, and
         * no local disk,
         * or target area marked as invalid,
         * or just got an io-error. */
        /* from drbd_make_request_common
         * or from bio_endio during read io-error recovery */

        /* so we can verify the handle in the answer packet
         * corresponding hlist_del is in _req_may_be_done() */
        hlist_add_head(&req->collision, ar_hash_slot(mdev, req->sector));

        set_bit(UNPLUG_REMOTE, &mdev->flags);

        D_ASSERT(req->rq_state & RQ_NET_PENDING);
        req->rq_state |= RQ_NET_QUEUED;
        req->w.cb = (req->rq_state & RQ_LOCAL_MASK)
            ? w_read_retry_remote
            : w_send_read_req;
        drbd_queue_work(&mdev->data.work, &req->w);
        break;

    case queue_for_net_write:
        /* assert something? */
        /* from drbd_make_request_common only */

        hlist_add_head(&req->collision, tl_hash_slot(mdev, req->sector));
        /* corresponding hlist_del is in _req_may_be_done() */

        /* NOTE
         * In case the req ended up on the transfer log before being
         * queued on the worker, it could lead to this request being
         * missed during cleanup after connection loss.
         * So we have to do both operations here,
         * within the same lock that protects the transfer log.
         *
         * _req_add_to_epoch(req); this has to be after the
         * _maybe_start_new_epoch(req); which happened in
         * drbd_make_request_common, because we now may set the bit
         * again ourselves to close the current epoch.
         *
         * Add req to the (now) current epoch (barrier). */

        /* otherwise we may lose an unplug, which may cause some remote
         * io-scheduler timeout to expire, increasing maximum latency,
         * hurting performance. */
        set_bit(UNPLUG_REMOTE, &mdev->flags);

        /* see drbd_make_request_common,
         * just after it grabs the req_lock */
        D_ASSERT(test_bit(CREATE_BARRIER, &mdev->flags) == 0);

        req->epoch = mdev->newest_tle->br_number;

        /* increment size of current epoch */
        mdev->newest_tle->n_writes++;

        /* queue work item to send data */
        D_ASSERT(req->rq_state & RQ_NET_PENDING);
        req->rq_state |= RQ_NET_QUEUED;
        req->w.cb =  w_send_dblock;
        drbd_queue_work(&mdev->data.work, &req->w);

        /* close the epoch, in case it outgrew the limit */
        if (mdev->newest_tle->n_writes >= mdev->net_conf->max_epoch_size)
            queue_barrier(mdev);

        break;

    case queue_for_send_oos:
        req->rq_state |= RQ_NET_QUEUED;
        req->w.cb =  w_send_oos;
        drbd_queue_work(&mdev->data.work, &req->w);
        break;

    case read_retry_remote_canceled:
    case send_canceled:
    case send_failed:
        /* real cleanup will be done from tl_clear.  just update flags
         * so it is no longer marked as on the worker queue */
        req->rq_state &= ~RQ_NET_QUEUED;
        /* if we did it right, tl_clear should be scheduled only after
         * this, so this should not be necessary! */
        _req_may_be_done_not_susp(req, m);
        break;

    case handed_over_to_network:
        /* assert something? */
        if (bio_data_dir(req->master_bio) == WRITE)
            atomic_add(req->size>>9, &mdev->ap_in_flight);

        if (bio_data_dir(req->master_bio) == WRITE &&
            mdev->net_conf->wire_protocol == DRBD_PROT_A) {
            /* this is what is dangerous about protocol A:
             * pretend it was successfully written on the peer. */
            if (req->rq_state & RQ_NET_PENDING) {
                dec_ap_pending(mdev);
                req->rq_state &= ~RQ_NET_PENDING;
                req->rq_state |= RQ_NET_OK;
            } /* else: neg-ack was faster... */
            /* it is still not yet RQ_NET_DONE until the
             * corresponding epoch barrier got acked as well,
             * so we know what to dirty on connection loss */
        }
        req->rq_state &= ~RQ_NET_QUEUED;
        req->rq_state |= RQ_NET_SENT;
        _req_may_be_done_not_susp(req, m);
        break;

    case oos_handed_to_network:
        /* Was not set PENDING, no longer QUEUED, so is now DONE
         * as far as this connection is concerned. */
        req->rq_state &= ~RQ_NET_QUEUED;
        req->rq_state |= RQ_NET_DONE;
        _req_may_be_done_not_susp(req, m);
        break;

    case connection_lost_while_pending:
        /* transfer log cleanup after connection loss */
        /* assert something? */
        if (req->rq_state & RQ_NET_PENDING)
            dec_ap_pending(mdev);
        req->rq_state &= ~(RQ_NET_OK|RQ_NET_PENDING);
        req->rq_state |= RQ_NET_DONE;
        if (req->rq_state & RQ_NET_SENT && req->rq_state & RQ_WRITE)
            atomic_sub(req->size>>9, &mdev->ap_in_flight);

        /* if it is still queued, we may not complete it here.
         * it will be canceled soon. */
        if (!(req->rq_state & RQ_NET_QUEUED))
            _req_may_be_done(req, m); /* Allowed while state.susp */
        break;

    case conflict_discarded_by_peer:
        /* for discarded conflicting writes of multiple primaries,
         * there is no need to keep anything in the tl, potential
         * node crashes are covered by the activity log. */
        if (what == conflict_discarded_by_peer)
            dev_alert(DEV, "Got DiscardAck packet %llus +%u!"
                  " DRBD is not a random data generator!\n",
                  (unsigned long long)req->sector, req->size);
        req->rq_state |= RQ_NET_DONE;
        /* fall through */
    case write_acked_by_peer_and_sis:
    case write_acked_by_peer:
        if (what == write_acked_by_peer_and_sis)
            req->rq_state |= RQ_NET_SIS;
        /* protocol C; successfully written on peer.
         * Nothing more to do here.
         * We want to keep the tl in place for all protocols, to cater
         * for volatile write-back caches on lower level devices. */

    case recv_acked_by_peer:
        /* protocol B; pretends to be successfully written on peer.
         * see also notes above in handed_over_to_network about
         * protocol != C */
        req->rq_state |= RQ_NET_OK;
        D_ASSERT(req->rq_state & RQ_NET_PENDING);
        dec_ap_pending(mdev);
        atomic_sub(req->size>>9, &mdev->ap_in_flight);
        req->rq_state &= ~RQ_NET_PENDING;
        _req_may_be_done_not_susp(req, m);
        break;

    case neg_acked:
        /* assert something? */
        if (req->rq_state & RQ_NET_PENDING) {
            dec_ap_pending(mdev);
            atomic_sub(req->size>>9, &mdev->ap_in_flight);
        }
        req->rq_state &= ~(RQ_NET_OK|RQ_NET_PENDING);

        req->rq_state |= RQ_NET_DONE;
        _req_may_be_done_not_susp(req, m);
        /* else: done by handed_over_to_network */
        break;

    case fail_frozen_disk_io:
        if (!(req->rq_state & RQ_LOCAL_COMPLETED))
            break;

        _req_may_be_done(req, m); /* Allowed while state.susp */
        break;

    case restart_frozen_disk_io:
        if (!(req->rq_state & RQ_LOCAL_COMPLETED))
            break;

        req->rq_state &= ~RQ_LOCAL_COMPLETED;

        rv = MR_READ;
        if (bio_data_dir(req->master_bio) == WRITE)
            rv = MR_WRITE;

        get_ldev(mdev);
        req->w.cb = w_restart_disk_io;
        drbd_queue_work(&mdev->data.work, &req->w);
        break;

    case resend:
        /* Simply complete (local only) READs. */
        if (!(req->rq_state & RQ_WRITE) && !req->w.cb) {
            _req_may_be_done(req, m);
            break;
        }

        /* If RQ_NET_OK is already set, we got a P_WRITE_ACK or P_RECV_ACK
           before the connection loss (B&C only); only P_BARRIER_ACK was missing.
           Trowing them out of the TL here by pretending we got a BARRIER_ACK
           We ensure that the peer was not rebooted */
        if (!(req->rq_state & RQ_NET_OK)) {
            if (req->w.cb) {
                drbd_queue_work(&mdev->data.work, &req->w);
                rv = req->rq_state & RQ_WRITE ? MR_WRITE : MR_READ;
            }
            break;
        }
        /* else, fall through to barrier_acked */

    case barrier_acked:
        if (!(req->rq_state & RQ_WRITE))
            break;

        if (req->rq_state & RQ_NET_PENDING) {
            /* barrier came in before all requests have been acked.
             * this is bad, because if the connection is lost now,
             * we won't be able to clean them up... */
            dev_err(DEV, "FIXME (barrier_acked but pending)\n");
            list_move(&req->tl_requests, &mdev->out_of_sequence_requests);
        }
        if ((req->rq_state & RQ_NET_MASK) != 0) {
            req->rq_state |= RQ_NET_DONE;
            if (mdev->net_conf->wire_protocol == DRBD_PROT_A)
                atomic_sub(req->size>>9, &mdev->ap_in_flight);
        }
        _req_may_be_done(req, m); /* Allowed while state.susp */
        break;

    case data_received:
        D_ASSERT(req->rq_state & RQ_NET_PENDING);
        dec_ap_pending(mdev);
        req->rq_state &= ~RQ_NET_PENDING;
        req->rq_state |= (RQ_NET_OK|RQ_NET_DONE);
        _req_may_be_done_not_susp(req, m);
        break;
    };

    return rv;
}

/* we may do a local read if:
 * - we are consistent (of course),
 * - or we are generally inconsistent,
 *   BUT we are still/already IN SYNC for this area.
 *   since size may be bigger than BM_BLOCK_SIZE,
 *   we may need to check several bits.
 */
static int drbd_may_do_local_read(struct drbd_conf *mdev, sector_t sector, int size)
{
    unsigned long sbnr, ebnr;
    sector_t esector, nr_sectors;

    if (mdev->state.disk == D_UP_TO_DATE)
        return 1;
    if (mdev->state.disk >= D_OUTDATED)
        return 0;
    if (mdev->state.disk <  D_INCONSISTENT)
        return 0;
    /* state.disk == D_INCONSISTENT   We will have a look at the BitMap */
    nr_sectors = drbd_get_capacity(mdev->this_bdev);
    esector = sector + (size >> 9) - 1;

    D_ASSERT(sector  < nr_sectors);
    D_ASSERT(esector < nr_sectors);

    sbnr = BM_SECT_TO_BIT(sector);
    ebnr = BM_SECT_TO_BIT(esector);

    return 0 == drbd_bm_count_bits(mdev, sbnr, ebnr);
}

static void maybe_pull_ahead(struct drbd_conf *mdev)
{
    int congested = 0;

    /* If I don't even have good local storage, we can not reasonably try
     * to pull ahead of the peer. We also need the local reference to make
     * sure mdev->act_log is there.
     * Note: caller has to make sure that net_conf is there.
     */
    if (!get_ldev_if_state(mdev, D_UP_TO_DATE))
        return;

    if (mdev->net_conf->cong_fill &&
        atomic_read(&mdev->ap_in_flight) >= mdev->net_conf->cong_fill) {
        dev_info(DEV, "Congestion-fill threshold reached\n");
        congested = 1;
    }

    if (mdev->act_log->used >= mdev->net_conf->cong_extents) {
        dev_info(DEV, "Congestion-extents threshold reached\n");
        congested = 1;
    }

    if (congested) {
        queue_barrier(mdev); /* last barrier, after mirrored writes */

        if (mdev->net_conf->on_congestion == OC_PULL_AHEAD)
            _drbd_set_state(_NS(mdev, conn, C_AHEAD), 0, NULL);
        else  /*mdev->net_conf->on_congestion == OC_DISCONNECT */
            _drbd_set_state(_NS(mdev, conn, C_DISCONNECTING), 0, NULL);
    }
    put_ldev(mdev);
}

static int drbd_make_request_common(struct drbd_conf *mdev, struct bio *bio, unsigned long start_time)
{
    const int rw = bio_rw(bio);
    const int size = bio->bi_size;
    const sector_t sector = bio->bi_sector;
    struct drbd_tl_epoch *b = NULL;
    struct drbd_request *req;
    int local, remote, send_oos = 0;
    int err = -EIO;
    int ret = 0;
    union drbd_state s;

    /* allocate outside of all locks; */
    req = drbd_req_new(mdev, bio);
    if (!req) {
        dec_ap_bio(mdev);
        /* only pass the error to the upper layers.
         * if user cannot handle io errors, that's not our business. */
        dev_err(DEV, "could not kmalloc() req\n");
        bio_endio(bio, -ENOMEM);
        return 0;
    }
    req->start_time = start_time;

    local = get_ldev(mdev);
    if (!local) {
        bio_put(req->private_bio); /* or we get a bio leak */
        req->private_bio = NULL;
    }
    if (rw == WRITE) {
        /* Need to replicate writes.  Unless it is an empty flush,
         * which is better mapped to a DRBD P_BARRIER packet,
         * also for drbd wire protocol compatibility reasons. */
        if (unlikely(size == 0)) {
            /* The only size==0 bios we expect are empty flushes. */
            D_ASSERT(bio->bi_rw & REQ_FLUSH);
            remote = 0;
        } else
            remote = 1;
    } else {
        /* READ || READA */
        if (local) {
            if (!drbd_may_do_local_read(mdev, sector, size)) {
                /* we could kick the syncer to
                 * sync this extent asap, wait for
                 * it, then continue locally.
                 * Or just issue the request remotely.
                 */
                local = 0;
                bio_put(req->private_bio);
                req->private_bio = NULL;
                put_ldev(mdev);
            }
        }
        remote = !local && mdev->state.pdsk >= D_UP_TO_DATE;
    }

    /* If we have a disk, but a READA request is mapped to remote,
     * we are R_PRIMARY, D_INCONSISTENT, SyncTarget.
     * Just fail that READA request right here.
     *
     * THINK: maybe fail all READA when not local?
     *        or make this configurable...
     *        if network is slow, READA won't do any good.
     */
    if (rw == READA && mdev->state.disk >= D_INCONSISTENT && !local) {
        err = -EWOULDBLOCK;
        goto fail_and_free_req;
    }

    /* For WRITES going to the local disk, grab a reference on the target
     * extent.  This waits for any resync activity in the corresponding
     * resync extent to finish, and, if necessary, pulls in the target
     * extent into the activity log, which involves further disk io because
     * of transactional on-disk meta data updates.
     * Empty flushes don't need to go into the activity log, they can only
     * flush data for pending writes which are already in there. */
    if (rw == WRITE && local && size
    && !test_bit(AL_SUSPENDED, &mdev->flags)) {
        req->rq_state |= RQ_IN_ACT_LOG;
        drbd_al_begin_io(mdev, sector);
    }

    s = mdev->state;
    remote = remote && drbd_should_do_remote(s);
    send_oos = rw == WRITE && drbd_should_send_oos(s);
    D_ASSERT(!(remote && send_oos));

    if (!(local || remote) && !is_susp(mdev->state)) {
        if (__ratelimit(&drbd_ratelimit_state))
            dev_err(DEV, "IO ERROR: neither local nor remote disk\n");
        goto fail_free_complete;
    }

    /* For WRITE request, we have to make sure that we have an
     * unused_spare_tle, in case we need to start a new epoch.
     * I try to be smart and avoid to pre-allocate always "just in case",
     * but there is a race between testing the bit and pointer outside the
     * spinlock, and grabbing the spinlock.
     * if we lost that race, we retry.  */
    if (rw == WRITE && (remote || send_oos) &&
        mdev->unused_spare_tle == NULL &&
        test_bit(CREATE_BARRIER, &mdev->flags)) {
allocate_barrier:
        b = kmalloc(sizeof(struct drbd_tl_epoch), GFP_NOIO);
        if (!b) {
            dev_err(DEV, "Failed to alloc barrier.\n");
            err = -ENOMEM;
            goto fail_free_complete;
        }
    }

    /* GOOD, everything prepared, grab the spin_lock */
    spin_lock_irq(&mdev->req_lock);

    if (is_susp(mdev->state)) {
        /* If we got suspended, use the retry mechanism of
           drbd_make_request() to restart processing of this
           bio. In the next call to drbd_make_request
           we sleep in inc_ap_bio() */
        ret = 1;
        spin_unlock_irq(&mdev->req_lock);
        goto fail_free_complete;
    }

    if (remote || send_oos) {
        remote = drbd_should_do_remote(mdev->state);
        send_oos = rw == WRITE && drbd_should_send_oos(mdev->state);
        D_ASSERT(!(remote && send_oos));

        if (!(remote || send_oos))
            dev_warn(DEV, "lost connection while grabbing the req_lock!\n");
        if (!(local || remote)) {
            dev_err(DEV, "IO ERROR: neither local nor remote disk\n");
            spin_unlock_irq(&mdev->req_lock);
            goto fail_free_complete;
        }
    }

    if (b && mdev->unused_spare_tle == NULL) {
        mdev->unused_spare_tle = b;
        b = NULL;
    }
    if (rw == WRITE && (remote || send_oos) &&
        mdev->unused_spare_tle == NULL &&
        test_bit(CREATE_BARRIER, &mdev->flags)) {
        /* someone closed the current epoch
         * while we were grabbing the spinlock */
        spin_unlock_irq(&mdev->req_lock);
        goto allocate_barrier;
    }


    /* Update disk stats */
    _drbd_start_io_acct(mdev, req, bio);

    /* _maybe_start_new_epoch(mdev);
     * If we need to generate a write barrier packet, we have to add the
     * new epoch (barrier) object, and queue the barrier packet for sending,
     * and queue the req's data after it _within the same lock_, otherwise
     * we have race conditions were the reorder domains could be mixed up.
     *
     * Even read requests may start a new epoch and queue the corresponding
     * barrier packet.  To get the write ordering right, we only have to
     * make sure that, if this is a write request and it triggered a
     * barrier packet, this request is queued within the same spinlock. */
    if ((remote || send_oos) && mdev->unused_spare_tle &&
        test_and_clear_bit(CREATE_BARRIER, &mdev->flags)) {
        _tl_add_barrier(mdev, mdev->unused_spare_tle);
        mdev->unused_spare_tle = NULL;
    } else {
        D_ASSERT(!(remote && rw == WRITE &&
               test_bit(CREATE_BARRIER, &mdev->flags)));
    }

    /* NOTE
     * Actually, 'local' may be wrong here already, since we may have failed
     * to write to the meta data, and may become wrong anytime because of
     * local io-error for some other request, which would lead to us
     * "detaching" the local disk.
     *
     * 'remote' may become wrong any time because the network could fail.
     *
     * This is a harmless race condition, though, since it is handled
     * correctly at the appropriate places; so it just defers the failure
     * of the respective operation.
     */

    /* mark them early for readability.
     * this just sets some state flags. */
    if (remote)
        _req_mod(req, to_be_send);
    if (local)
        _req_mod(req, to_be_submitted);

    /* check this request on the collision detection hash tables.
     * if we have a conflict, just complete it here.
     * THINK do we want to check reads, too? (I don't think so...) */
    if (rw == WRITE && _req_conflicts(req))
        goto fail_conflicting;

    /* no point in adding empty flushes to the transfer log,
     * they are mapped to drbd barriers already. */
    if (likely(size!=0))
        list_add_tail(&req->tl_requests, &mdev->newest_tle->requests);

    /* NOTE remote first: to get the concurrent write detection right,
     * we must register the request before start of local IO.  */
    if (remote) {
        /* either WRITE and C_CONNECTED,
         * or READ, and no local disk,
         * or READ, but not in sync.
         */
        _req_mod(req, (rw == WRITE)
                ? queue_for_net_write
                : queue_for_net_read);
    }
    if (send_oos && drbd_set_out_of_sync(mdev, sector, size))
        _req_mod(req, queue_for_send_oos);

    if (remote &&
        mdev->net_conf->on_congestion != OC_BLOCK && mdev->agreed_pro_version >= 96)
        maybe_pull_ahead(mdev);

    /* If this was a flush, queue a drbd barrier/start a new epoch.
     * Unless the current epoch was empty anyways, or we are not currently
     * replicating, in which case there is no point. */
    if (unlikely(bio->bi_rw & REQ_FLUSH)
        && mdev->newest_tle->n_writes
        && drbd_should_do_remote(mdev->state))
        queue_barrier(mdev);

    spin_unlock_irq(&mdev->req_lock);
    kfree(b); /* if someone else has beaten us to it... */

    if (local) {
        req->private_bio->bi_bdev = mdev->ldev->backing_bdev;

        /* State may have changed since we grabbed our reference on the
         * mdev->ldev member. Double check, and short-circuit to endio.
         * In case the last activity log transaction failed to get on
         * stable storage, and this is a WRITE, we may not even submit
         * this bio. */
        if (get_ldev(mdev)) {
            if (drbd_insert_fault(mdev,   rw == WRITE ? DRBD_FAULT_DT_WR
                            : rw == READ  ? DRBD_FAULT_DT_RD
                            :               DRBD_FAULT_DT_RA))
                bio_endio(req->private_bio, -EIO);
            else
                generic_make_request(req->private_bio);
            put_ldev(mdev);
        } else
            bio_endio(req->private_bio, -EIO);
    }

    return 0;

fail_conflicting:
    /* this is a conflicting request.
     * even though it may have been only _partially_
     * overlapping with one of the currently pending requests,
     * without even submitting or sending it, we will
     * pretend that it was successfully served right now.
     */
    _drbd_end_io_acct(mdev, req);
    spin_unlock_irq(&mdev->req_lock);
    if (remote)
        dec_ap_pending(mdev);
    /* THINK: do we want to fail it (-EIO), or pretend success?
     * this pretends success. */
    err = 0;

fail_free_complete:
    if (req->rq_state & RQ_IN_ACT_LOG)
        drbd_al_complete_io(mdev, sector);
fail_and_free_req:
    if (local) {
        bio_put(req->private_bio);
        req->private_bio = NULL;
        put_ldev(mdev);
    }
    if (!ret)
        bio_endio(bio, err);

    drbd_req_free(req);
    dec_ap_bio(mdev);
    kfree(b);

    return ret;
}

/* helper function for drbd_make_request
 * if we can determine just by the mdev (state) that this request will fail,
 * return 1
 * otherwise return 0
 */
static int drbd_fail_request_early(struct drbd_conf *mdev, int is_write)
{
    if (mdev->state.role != R_PRIMARY &&
        (!allow_oos || is_write)) {
        if (__ratelimit(&drbd_ratelimit_state)) {
            dev_err(DEV, "Process %s[%u] tried to %s; "
                "since we are not in Primary state, "
                "we cannot allow this\n",
                current->comm, current->pid,
                is_write ? "WRITE" : "READ");
        }
        return 1;
    }

    return 0;
}

void drbd_make_request(struct request_queue *q, struct bio *bio)
{
    unsigned int s_enr, e_enr;
    struct drbd_conf *mdev = (struct drbd_conf *) q->queuedata;
    unsigned long start_time;

    if (drbd_fail_request_early(mdev, bio_data_dir(bio) & WRITE)) {
        bio_endio(bio, -EPERM);
        return;
    }

    start_time = jiffies;

    /*
     * what we "blindly" assume:
     */
    D_ASSERT((bio->bi_size & 0x1ff) == 0);

    /* to make some things easier, force alignment of requests within the
     * granularity of our hash tables */
    s_enr = bio->bi_sector >> HT_SHIFT;
    e_enr = bio->bi_size ? (bio->bi_sector+(bio->bi_size>>9)-1) >> HT_SHIFT : s_enr;

    if (likely(s_enr == e_enr)) {
        do {
            inc_ap_bio(mdev, 1);
        } while (drbd_make_request_common(mdev, bio, start_time));
        return;
    }

    /* can this bio be split generically?
     * Maybe add our own split-arbitrary-bios function. */
    if (bio->bi_vcnt != 1 || bio->bi_idx != 0 || bio->bi_size > DRBD_MAX_BIO_SIZE) {
        /* rather error out here than BUG in bio_split */
        dev_err(DEV, "bio would need to, but cannot, be split: "
            "(vcnt=%u,idx=%u,size=%u,sector=%llu)\n",
            bio->bi_vcnt, bio->bi_idx, bio->bi_size,
            (unsigned long long)bio->bi_sector);
        bio_endio(bio, -EINVAL);
    } else {
        /* This bio crosses some boundary, so we have to split it. */
        struct bio_pair *bp;
        /* works for the "do not cross hash slot boundaries" case
         * e.g. sector 262269, size 4096
         * s_enr = 262269 >> 6 = 4097
         * e_enr = (262269+8-1) >> 6 = 4098
         * HT_SHIFT = 6
         * sps = 64, mask = 63
         * first_sectors = 64 - (262269 & 63) = 3
         */
        const sector_t sect = bio->bi_sector;
        const int sps = 1 << HT_SHIFT; /* sectors per slot */
        const int mask = sps - 1;
        const sector_t first_sectors = sps - (sect & mask);
        bp = bio_split(bio, first_sectors);

        /* we need to get a "reference count" (ap_bio_cnt)
         * to avoid races with the disconnect/reconnect/suspend code.
         * In case we need to split the bio here, we need to get three references
         * atomically, otherwise we might deadlock when trying to submit the
         * second one! */
        inc_ap_bio(mdev, 3);

        D_ASSERT(e_enr == s_enr + 1);

        while (drbd_make_request_common(mdev, &bp->bio1, start_time))
            inc_ap_bio(mdev, 1);

        while (drbd_make_request_common(mdev, &bp->bio2, start_time))
            inc_ap_bio(mdev, 1);

        dec_ap_bio(mdev);

        bio_pair_release(bp);
    }
}

/* This is called by bio_add_page().  With this function we reduce
 * the number of BIOs that span over multiple DRBD_MAX_BIO_SIZEs
 * units (was AL_EXTENTs).
 *
 * we do the calculation within the lower 32bit of the byte offsets,
 * since we don't care for actual offset, but only check whether it
 * would cross "activity log extent" boundaries.
 *
 * As long as the BIO is empty we have to allow at least one bvec,
 * regardless of size and offset.  so the resulting bio may still
 * cross extent boundaries.  those are dealt with (bio_split) in
 * drbd_make_request.
 */
int drbd_merge_bvec(struct request_queue *q, struct bvec_merge_data *bvm, struct bio_vec *bvec)
{
    struct drbd_conf *mdev = (struct drbd_conf *) q->queuedata;
    unsigned int bio_offset =
        (unsigned int)bvm->bi_sector << 9; /* 32 bit */
    unsigned int bio_size = bvm->bi_size;
    int limit, backing_limit;

    limit = DRBD_MAX_BIO_SIZE
          - ((bio_offset & (DRBD_MAX_BIO_SIZE-1)) + bio_size);
    if (limit < 0)
        limit = 0;
    if (bio_size == 0) {
        if (limit <= bvec->bv_len)
            limit = bvec->bv_len;
    } else if (limit && get_ldev(mdev)) {
        struct request_queue * const b =
            mdev->ldev->backing_bdev->bd_disk->queue;
        if (b->merge_bvec_fn) {
            backing_limit = b->merge_bvec_fn(b, bvm, bvec);
            limit = min(limit, backing_limit);
        }
        put_ldev(mdev);
    }
    return limit;
}

void request_timer_fn(unsigned long data)
{
    struct drbd_conf *mdev = (struct drbd_conf *) data;
    struct drbd_request *req; /* oldest request */
    struct list_head *le;
    unsigned long ent = 0, dt = 0, et, nt; /* effective timeout = ko_count * timeout */
    unsigned long now;

    if (get_net_conf(mdev)) {
        if (mdev->state.conn >= C_WF_REPORT_PARAMS)
            ent = mdev->net_conf->timeout*HZ/10
                * mdev->net_conf->ko_count;
        put_net_conf(mdev);
    }
    if (get_ldev(mdev)) { /* implicit state.disk >= D_INCONSISTENT */
        dt = mdev->ldev->dc.disk_timeout * HZ / 10;
        put_ldev(mdev);
    }
    et = min_not_zero(dt, ent);

    if (!et)
        return; /* Recurring timer stopped */

    now = jiffies;

    spin_lock_irq(&mdev->req_lock);
    le = &mdev->oldest_tle->requests;
    if (list_empty(le)) {
        spin_unlock_irq(&mdev->req_lock);
        mod_timer(&mdev->request_timer, now + et);
        return;
    }

    le = le->prev;
    req = list_entry(le, struct drbd_request, tl_requests);

    /* The request is considered timed out, if
     * - we have some effective timeout from the configuration,
     *   with above state restrictions applied,
     * - the oldest request is waiting for a response from the network
     *   resp. the local disk,
     * - the oldest request is in fact older than the effective timeout,
     * - the connection was established (resp. disk was attached)
     *   for longer than the timeout already.
     * Note that for 32bit jiffies and very stable connections/disks,
     * we may have a wrap around, which is catched by
     *   !time_in_range(now, last_..._jif, last_..._jif + timeout).
     *
     * Side effect: once per 32bit wrap-around interval, which means every
     * ~198 days with 250 HZ, we have a window where the timeout would need
     * to expire twice (worst case) to become effective. Good enough.
     */
    if (ent && req->rq_state & RQ_NET_PENDING &&
         time_after(now, req->start_time + ent) &&
        !time_in_range(now, mdev->last_reconnect_jif, mdev->last_reconnect_jif + ent)) {
        dev_warn(DEV, "Remote failed to finish a request within ko-count * timeout\n");
        _drbd_set_state(_NS(mdev, conn, C_TIMEOUT), CS_VERBOSE | CS_HARD, NULL);
    }
    if (dt && req->rq_state & RQ_LOCAL_PENDING &&
         time_after(now, req->start_time + dt) &&
        !time_in_range(now, mdev->last_reattach_jif, mdev->last_reattach_jif + dt)) {
        dev_warn(DEV, "Local backing device failed to meet the disk-timeout\n");
        __drbd_chk_io_error(mdev, DRBD_FORCE_DETACH);
    }
    nt = (time_after(now, req->start_time + et) ? now : req->start_time) + et;
    spin_unlock_irq(&mdev->req_lock);
    mod_timer(&mdev->request_timer, nt);
}