snap.c

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#include <linux/ceph/ceph_debug.h>

#include <linux/sort.h>
#include <linux/slab.h>

#include "super.h"
#include "mds_client.h"

#include <linux/ceph/decode.h>

/*
 * Snapshots in ceph are driven in large part by cooperation from the
 * client.  In contrast to local file systems or file servers that
 * implement snapshots at a single point in the system, ceph's
 * distributed access to storage requires clients to help decide
 * whether a write logically occurs before or after a recently created
 * snapshot.
 *
 * This provides a perfect instantanous client-wide snapshot.  Between
 * clients, however, snapshots may appear to be applied at slightly
 * different points in time, depending on delays in delivering the
 * snapshot notification.
 *
 * Snapshots are _not_ file system-wide.  Instead, each snapshot
 * applies to the subdirectory nested beneath some directory.  This
 * effectively divides the hierarchy into multiple "realms," where all
 * of the files contained by each realm share the same set of
 * snapshots.  An individual realm's snap set contains snapshots
 * explicitly created on that realm, as well as any snaps in its
 * parent's snap set _after_ the point at which the parent became it's
 * parent (due to, say, a rename).  Similarly, snaps from prior parents
 * during the time intervals during which they were the parent are included.
 *
 * The client is spared most of this detail, fortunately... it must only
 * maintains a hierarchy of realms reflecting the current parent/child
 * realm relationship, and for each realm has an explicit list of snaps
 * inherited from prior parents.
 *
 * A snap_realm struct is maintained for realms containing every inode
 * with an open cap in the system.  (The needed snap realm information is
 * provided by the MDS whenever a cap is issued, i.e., on open.)  A 'seq'
 * version number is used to ensure that as realm parameters change (new
 * snapshot, new parent, etc.) the client's realm hierarchy is updated.
 *
 * The realm hierarchy drives the generation of a 'snap context' for each
 * realm, which simply lists the resulting set of snaps for the realm.  This
 * is attached to any writes sent to OSDs.
 */
/*
 * Unfortunately error handling is a bit mixed here.  If we get a snap
 * update, but don't have enough memory to update our realm hierarchy,
 * it's not clear what we can do about it (besides complaining to the
 * console).
 */


/*
 * increase ref count for the realm
 *
 * caller must hold snap_rwsem for write.
 */
void ceph_get_snap_realm(struct ceph_mds_client *mdsc,
             struct ceph_snap_realm *realm)
{
    dout("get_realm %p %d -> %d\n", realm,
         atomic_read(&realm->nref), atomic_read(&realm->nref)+1);
    /*
     * since we _only_ increment realm refs or empty the empty
     * list with snap_rwsem held, adjusting the empty list here is
     * safe.  we do need to protect against concurrent empty list
     * additions, however.
     */
    if (atomic_read(&realm->nref) == 0) {
        spin_lock(&mdsc->snap_empty_lock);
        list_del_init(&realm->empty_item);
        spin_unlock(&mdsc->snap_empty_lock);
    }

    atomic_inc(&realm->nref);
}

static void __insert_snap_realm(struct rb_root *root,
                struct ceph_snap_realm *new)
{
    struct rb_node **p = &root->rb_node;
    struct rb_node *parent = NULL;
    struct ceph_snap_realm *r = NULL;

    while (*p) {
        parent = *p;
        r = rb_entry(parent, struct ceph_snap_realm, node);
        if (new->ino < r->ino)
            p = &(*p)->rb_left;
        else if (new->ino > r->ino)
            p = &(*p)->rb_right;
        else
            BUG();
    }

    rb_link_node(&new->node, parent, p);
    rb_insert_color(&new->node, root);
}

/*
 * create and get the realm rooted at @ino and bump its ref count.
 *
 * caller must hold snap_rwsem for write.
 */
static struct ceph_snap_realm *ceph_create_snap_realm(
    struct ceph_mds_client *mdsc,
    u64 ino)
{
    struct ceph_snap_realm *realm;

    realm = kzalloc(sizeof(*realm), GFP_NOFS);
    if (!realm)
        return ERR_PTR(-ENOMEM);

    atomic_set(&realm->nref, 0);    /* tree does not take a ref */
    realm->ino = ino;
    INIT_LIST_HEAD(&realm->children);
    INIT_LIST_HEAD(&realm->child_item);
    INIT_LIST_HEAD(&realm->empty_item);
    INIT_LIST_HEAD(&realm->dirty_item);
    INIT_LIST_HEAD(&realm->inodes_with_caps);
    spin_lock_init(&realm->inodes_with_caps_lock);
    __insert_snap_realm(&mdsc->snap_realms, realm);
    dout("create_snap_realm %llx %p\n", realm->ino, realm);
    return realm;
}

/*
 * lookup the realm rooted at @ino.
 *
 * caller must hold snap_rwsem for write.
 */
struct ceph_snap_realm *ceph_lookup_snap_realm(struct ceph_mds_client *mdsc,
                           u64 ino)
{
    struct rb_node *n = mdsc->snap_realms.rb_node;
    struct ceph_snap_realm *r;

    while (n) {
        r = rb_entry(n, struct ceph_snap_realm, node);
        if (ino < r->ino)
            n = n->rb_left;
        else if (ino > r->ino)
            n = n->rb_right;
        else {
            dout("lookup_snap_realm %llx %p\n", r->ino, r);
            return r;
        }
    }
    return NULL;
}

static void __put_snap_realm(struct ceph_mds_client *mdsc,
                 struct ceph_snap_realm *realm);

/*
 * called with snap_rwsem (write)
 */
static void __destroy_snap_realm(struct ceph_mds_client *mdsc,
                 struct ceph_snap_realm *realm)
{
    dout("__destroy_snap_realm %p %llx\n", realm, realm->ino);

    rb_erase(&realm->node, &mdsc->snap_realms);

    if (realm->parent) {
        list_del_init(&realm->child_item);
        __put_snap_realm(mdsc, realm->parent);
    }

    kfree(realm->prior_parent_snaps);
    kfree(realm->snaps);
    ceph_put_snap_context(realm->cached_context);
    kfree(realm);
}

/*
 * caller holds snap_rwsem (write)
 */
static void __put_snap_realm(struct ceph_mds_client *mdsc,
                 struct ceph_snap_realm *realm)
{
    dout("__put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
         atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
    if (atomic_dec_and_test(&realm->nref))
        __destroy_snap_realm(mdsc, realm);
}

/*
 * caller needn't hold any locks
 */
void ceph_put_snap_realm(struct ceph_mds_client *mdsc,
             struct ceph_snap_realm *realm)
{
    dout("put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
         atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
    if (!atomic_dec_and_test(&realm->nref))
        return;

    if (down_write_trylock(&mdsc->snap_rwsem)) {
        __destroy_snap_realm(mdsc, realm);
        up_write(&mdsc->snap_rwsem);
    } else {
        spin_lock(&mdsc->snap_empty_lock);
        list_add(&realm->empty_item, &mdsc->snap_empty);
        spin_unlock(&mdsc->snap_empty_lock);
    }
}

/*
 * Clean up any realms whose ref counts have dropped to zero.  Note
 * that this does not include realms who were created but not yet
 * used.
 *
 * Called under snap_rwsem (write)
 */
static void __cleanup_empty_realms(struct ceph_mds_client *mdsc)
{
    struct ceph_snap_realm *realm;

    spin_lock(&mdsc->snap_empty_lock);
    while (!list_empty(&mdsc->snap_empty)) {
        realm = list_first_entry(&mdsc->snap_empty,
                   struct ceph_snap_realm, empty_item);
        list_del(&realm->empty_item);
        spin_unlock(&mdsc->snap_empty_lock);
        __destroy_snap_realm(mdsc, realm);
        spin_lock(&mdsc->snap_empty_lock);
    }
    spin_unlock(&mdsc->snap_empty_lock);
}

void ceph_cleanup_empty_realms(struct ceph_mds_client *mdsc)
{
    down_write(&mdsc->snap_rwsem);
    __cleanup_empty_realms(mdsc);
    up_write(&mdsc->snap_rwsem);
}

/*
 * adjust the parent realm of a given @realm.  adjust child list, and parent
 * pointers, and ref counts appropriately.
 *
 * return true if parent was changed, 0 if unchanged, <0 on error.
 *
 * caller must hold snap_rwsem for write.
 */
static int adjust_snap_realm_parent(struct ceph_mds_client *mdsc,
                    struct ceph_snap_realm *realm,
                    u64 parentino)
{
    struct ceph_snap_realm *parent;

    if (realm->parent_ino == parentino)
        return 0;

    parent = ceph_lookup_snap_realm(mdsc, parentino);
    if (!parent) {
        parent = ceph_create_snap_realm(mdsc, parentino);
        if (IS_ERR(parent))
            return PTR_ERR(parent);
    }
    dout("adjust_snap_realm_parent %llx %p: %llx %p -> %llx %p\n",
         realm->ino, realm, realm->parent_ino, realm->parent,
         parentino, parent);
    if (realm->parent) {
        list_del_init(&realm->child_item);
        ceph_put_snap_realm(mdsc, realm->parent);
    }
    realm->parent_ino = parentino;
    realm->parent = parent;
    ceph_get_snap_realm(mdsc, parent);
    list_add(&realm->child_item, &parent->children);
    return 1;
}


static int cmpu64_rev(const void *a, const void *b)
{
    if (*(u64 *)a < *(u64 *)b)
        return 1;
    if (*(u64 *)a > *(u64 *)b)
        return -1;
    return 0;
}

/*
 * build the snap context for a given realm.
 */
static int build_snap_context(struct ceph_snap_realm *realm)
{
    struct ceph_snap_realm *parent = realm->parent;
    struct ceph_snap_context *snapc;
    int err = 0;
    u32 num = realm->num_prior_parent_snaps + realm->num_snaps;

    /*
     * build parent context, if it hasn't been built.
     * conservatively estimate that all parent snaps might be
     * included by us.
     */
    if (parent) {
        if (!parent->cached_context) {
            err = build_snap_context(parent);
            if (err)
                goto fail;
        }
        num += parent->cached_context->num_snaps;
    }

    /* do i actually need to update?  not if my context seq
       matches realm seq, and my parents' does to.  (this works
       because we rebuild_snap_realms() works _downward_ in
       hierarchy after each update.) */
    if (realm->cached_context &&
        realm->cached_context->seq == realm->seq &&
        (!parent ||
         realm->cached_context->seq >= parent->cached_context->seq)) {
        dout("build_snap_context %llx %p: %p seq %lld (%u snaps)"
             " (unchanged)\n",
             realm->ino, realm, realm->cached_context,
             realm->cached_context->seq,
             (unsigned int) realm->cached_context->num_snaps);
        return 0;
    }

    /* alloc new snap context */
    err = -ENOMEM;
    if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
        goto fail;
    snapc = kzalloc(sizeof(*snapc) + num*sizeof(u64), GFP_NOFS);
    if (!snapc)
        goto fail;
    atomic_set(&snapc->nref, 1);

    /* build (reverse sorted) snap vector */
    num = 0;
    snapc->seq = realm->seq;
    if (parent) {
        u32 i;

        /* include any of parent's snaps occurring _after_ my
           parent became my parent */
        for (i = 0; i < parent->cached_context->num_snaps; i++)
            if (parent->cached_context->snaps[i] >=
                realm->parent_since)
                snapc->snaps[num++] =
                    parent->cached_context->snaps[i];
        if (parent->cached_context->seq > snapc->seq)
            snapc->seq = parent->cached_context->seq;
    }
    memcpy(snapc->snaps + num, realm->snaps,
           sizeof(u64)*realm->num_snaps);
    num += realm->num_snaps;
    memcpy(snapc->snaps + num, realm->prior_parent_snaps,
           sizeof(u64)*realm->num_prior_parent_snaps);
    num += realm->num_prior_parent_snaps;

    sort(snapc->snaps, num, sizeof(u64), cmpu64_rev, NULL);
    snapc->num_snaps = num;
    dout("build_snap_context %llx %p: %p seq %lld (%u snaps)\n",
         realm->ino, realm, snapc, snapc->seq,
         (unsigned int) snapc->num_snaps);

    if (realm->cached_context)
        ceph_put_snap_context(realm->cached_context);
    realm->cached_context = snapc;
    return 0;

fail:
    /*
     * if we fail, clear old (incorrect) cached_context... hopefully
     * we'll have better luck building it later
     */
    if (realm->cached_context) {
        ceph_put_snap_context(realm->cached_context);
        realm->cached_context = NULL;
    }
    pr_err("build_snap_context %llx %p fail %d\n", realm->ino,
           realm, err);
    return err;
}

/*
 * rebuild snap context for the given realm and all of its children.
 */
static void rebuild_snap_realms(struct ceph_snap_realm *realm)
{
    struct ceph_snap_realm *child;

    dout("rebuild_snap_realms %llx %p\n", realm->ino, realm);
    build_snap_context(realm);

    list_for_each_entry(child, &realm->children, child_item)
        rebuild_snap_realms(child);
}


/*
 * helper to allocate and decode an array of snapids.  free prior
 * instance, if any.
 */
static int dup_array(u64 **dst, __le64 *src, u32 num)
{
    u32 i;

    kfree(*dst);
    if (num) {
        *dst = kcalloc(num, sizeof(u64), GFP_NOFS);
        if (!*dst)
            return -ENOMEM;
        for (i = 0; i < num; i++)
            (*dst)[i] = get_unaligned_le64(src + i);
    } else {
        *dst = NULL;
    }
    return 0;
}


/*
 * When a snapshot is applied, the size/mtime inode metadata is queued
 * in a ceph_cap_snap (one for each snapshot) until writeback
 * completes and the metadata can be flushed back to the MDS.
 *
 * However, if a (sync) write is currently in-progress when we apply
 * the snapshot, we have to wait until the write succeeds or fails
 * (and a final size/mtime is known).  In this case the
 * cap_snap->writing = 1, and is said to be "pending."  When the write
 * finishes, we __ceph_finish_cap_snap().
 *
 * Caller must hold snap_rwsem for read (i.e., the realm topology won't
 * change).
 */
void ceph_queue_cap_snap(struct ceph_inode_info *ci)
{
    struct inode *inode = &ci->vfs_inode;
    struct ceph_cap_snap *capsnap;
    int used, dirty;

    capsnap = kzalloc(sizeof(*capsnap), GFP_NOFS);
    if (!capsnap) {
        pr_err("ENOMEM allocating ceph_cap_snap on %p\n", inode);
        return;
    }

    spin_lock(&ci->i_ceph_lock);
    used = __ceph_caps_used(ci);
    dirty = __ceph_caps_dirty(ci);

    /*
     * If there is a write in progress, treat that as a dirty Fw,
     * even though it hasn't completed yet; by the time we finish
     * up this capsnap it will be.
     */
    if (used & CEPH_CAP_FILE_WR)
        dirty |= CEPH_CAP_FILE_WR;

    if (__ceph_have_pending_cap_snap(ci)) {
        /* there is no point in queuing multiple "pending" cap_snaps,
           as no new writes are allowed to start when pending, so any
           writes in progress now were started before the previous
           cap_snap.  lucky us. */
        dout("queue_cap_snap %p already pending\n", inode);
        kfree(capsnap);
    } else if (dirty & (CEPH_CAP_AUTH_EXCL|CEPH_CAP_XATTR_EXCL|
                CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_WR)) {
        struct ceph_snap_context *snapc = ci->i_head_snapc;

        /*
         * if we are a sync write, we may need to go to the snaprealm
         * to get the current snapc.
         */
        if (!snapc)
            snapc = ci->i_snap_realm->cached_context;

        dout("queue_cap_snap %p cap_snap %p queuing under %p %s\n",
             inode, capsnap, snapc, ceph_cap_string(dirty));
        ihold(inode);

        atomic_set(&capsnap->nref, 1);
        capsnap->ci = ci;
        INIT_LIST_HEAD(&capsnap->ci_item);
        INIT_LIST_HEAD(&capsnap->flushing_item);

        capsnap->follows = snapc->seq;
        capsnap->issued = __ceph_caps_issued(ci, NULL);
        capsnap->dirty = dirty;

        capsnap->mode = inode->i_mode;
        capsnap->uid = inode->i_uid;
        capsnap->gid = inode->i_gid;

        if (dirty & CEPH_CAP_XATTR_EXCL) {
            __ceph_build_xattrs_blob(ci);
            capsnap->xattr_blob =
                ceph_buffer_get(ci->i_xattrs.blob);
            capsnap->xattr_version = ci->i_xattrs.version;
        } else {
            capsnap->xattr_blob = NULL;
            capsnap->xattr_version = 0;
        }

        /* dirty page count moved from _head to this cap_snap;
           all subsequent writes page dirties occur _after_ this
           snapshot. */
        capsnap->dirty_pages = ci->i_wrbuffer_ref_head;
        ci->i_wrbuffer_ref_head = 0;
        capsnap->context = snapc;
        ci->i_head_snapc =
            ceph_get_snap_context(ci->i_snap_realm->cached_context);
        dout(" new snapc is %p\n", ci->i_head_snapc);
        list_add_tail(&capsnap->ci_item, &ci->i_cap_snaps);

        if (used & CEPH_CAP_FILE_WR) {
            dout("queue_cap_snap %p cap_snap %p snapc %p"
                 " seq %llu used WR, now pending\n", inode,
                 capsnap, snapc, snapc->seq);
            capsnap->writing = 1;
        } else {
            /* note mtime, size NOW. */
            __ceph_finish_cap_snap(ci, capsnap);
        }
    } else {
        dout("queue_cap_snap %p nothing dirty|writing\n", inode);
        kfree(capsnap);
    }

    spin_unlock(&ci->i_ceph_lock);
}

/*
 * Finalize the size, mtime for a cap_snap.. that is, settle on final values
 * to be used for the snapshot, to be flushed back to the mds.
 *
 * If capsnap can now be flushed, add to snap_flush list, and return 1.
 *
 * Caller must hold i_ceph_lock.
 */
int __ceph_finish_cap_snap(struct ceph_inode_info *ci,
                struct ceph_cap_snap *capsnap)
{
    struct inode *inode = &ci->vfs_inode;
    struct ceph_mds_client *mdsc = ceph_sb_to_client(inode->i_sb)->mdsc;

    BUG_ON(capsnap->writing);
    capsnap->size = inode->i_size;
    capsnap->mtime = inode->i_mtime;
    capsnap->atime = inode->i_atime;
    capsnap->ctime = inode->i_ctime;
    capsnap->time_warp_seq = ci->i_time_warp_seq;
    if (capsnap->dirty_pages) {
        dout("finish_cap_snap %p cap_snap %p snapc %p %llu %s s=%llu "
             "still has %d dirty pages\n", inode, capsnap,
             capsnap->context, capsnap->context->seq,
             ceph_cap_string(capsnap->dirty), capsnap->size,
             capsnap->dirty_pages);
        return 0;
    }
    dout("finish_cap_snap %p cap_snap %p snapc %p %llu %s s=%llu\n",
         inode, capsnap, capsnap->context,
         capsnap->context->seq, ceph_cap_string(capsnap->dirty),
         capsnap->size);

    spin_lock(&mdsc->snap_flush_lock);
    list_add_tail(&ci->i_snap_flush_item, &mdsc->snap_flush_list);
    spin_unlock(&mdsc->snap_flush_lock);
    return 1;  /* caller may want to ceph_flush_snaps */
}

/*
 * Queue cap_snaps for snap writeback for this realm and its children.
 * Called under snap_rwsem, so realm topology won't change.
 */
static void queue_realm_cap_snaps(struct ceph_snap_realm *realm)
{
    struct ceph_inode_info *ci;
    struct inode *lastinode = NULL;
    struct ceph_snap_realm *child;

    dout("queue_realm_cap_snaps %p %llx inodes\n", realm, realm->ino);

    spin_lock(&realm->inodes_with_caps_lock);
    list_for_each_entry(ci, &realm->inodes_with_caps,
                i_snap_realm_item) {
        struct inode *inode = igrab(&ci->vfs_inode);
        if (!inode)
            continue;
        spin_unlock(&realm->inodes_with_caps_lock);
        if (lastinode)
            iput(lastinode);
        lastinode = inode;
        ceph_queue_cap_snap(ci);
        spin_lock(&realm->inodes_with_caps_lock);
    }
    spin_unlock(&realm->inodes_with_caps_lock);
    if (lastinode)
        iput(lastinode);

    list_for_each_entry(child, &realm->children, child_item) {
        dout("queue_realm_cap_snaps %p %llx queue child %p %llx\n",
             realm, realm->ino, child, child->ino);
        list_del_init(&child->dirty_item);
        list_add(&child->dirty_item, &realm->dirty_item);
    }

    list_del_init(&realm->dirty_item);
    dout("queue_realm_cap_snaps %p %llx done\n", realm, realm->ino);
}

/*
 * Parse and apply a snapblob "snap trace" from the MDS.  This specifies
 * the snap realm parameters from a given realm and all of its ancestors,
 * up to the root.
 *
 * Caller must hold snap_rwsem for write.
 */
int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
               void *p, void *e, bool deletion)
{
    struct ceph_mds_snap_realm *ri;    /* encoded */
    __le64 *snaps;                     /* encoded */
    __le64 *prior_parent_snaps;        /* encoded */
    struct ceph_snap_realm *realm;
    int invalidate = 0;
    int err = -ENOMEM;
    LIST_HEAD(dirty_realms);

    dout("update_snap_trace deletion=%d\n", deletion);
more:
    ceph_decode_need(&p, e, sizeof(*ri), bad);
    ri = p;
    p += sizeof(*ri);
    ceph_decode_need(&p, e, sizeof(u64)*(le32_to_cpu(ri->num_snaps) +
                le32_to_cpu(ri->num_prior_parent_snaps)), bad);
    snaps = p;
    p += sizeof(u64) * le32_to_cpu(ri->num_snaps);
    prior_parent_snaps = p;
    p += sizeof(u64) * le32_to_cpu(ri->num_prior_parent_snaps);

    realm = ceph_lookup_snap_realm(mdsc, le64_to_cpu(ri->ino));
    if (!realm) {
        realm = ceph_create_snap_realm(mdsc, le64_to_cpu(ri->ino));
        if (IS_ERR(realm)) {
            err = PTR_ERR(realm);
            goto fail;
        }
    }

    /* ensure the parent is correct */
    err = adjust_snap_realm_parent(mdsc, realm, le64_to_cpu(ri->parent));
    if (err < 0)
        goto fail;
    invalidate += err;

    if (le64_to_cpu(ri->seq) > realm->seq) {
        dout("update_snap_trace updating %llx %p %lld -> %lld\n",
             realm->ino, realm, realm->seq, le64_to_cpu(ri->seq));
        /* update realm parameters, snap lists */
        realm->seq = le64_to_cpu(ri->seq);
        realm->created = le64_to_cpu(ri->created);
        realm->parent_since = le64_to_cpu(ri->parent_since);

        realm->num_snaps = le32_to_cpu(ri->num_snaps);
        err = dup_array(&realm->snaps, snaps, realm->num_snaps);
        if (err < 0)
            goto fail;

        realm->num_prior_parent_snaps =
            le32_to_cpu(ri->num_prior_parent_snaps);
        err = dup_array(&realm->prior_parent_snaps, prior_parent_snaps,
                realm->num_prior_parent_snaps);
        if (err < 0)
            goto fail;

        /* queue realm for cap_snap creation */
        list_add(&realm->dirty_item, &dirty_realms);

        invalidate = 1;
    } else if (!realm->cached_context) {
        dout("update_snap_trace %llx %p seq %lld new\n",
             realm->ino, realm, realm->seq);
        invalidate = 1;
    } else {
        dout("update_snap_trace %llx %p seq %lld unchanged\n",
             realm->ino, realm, realm->seq);
    }

    dout("done with %llx %p, invalidated=%d, %p %p\n", realm->ino,
         realm, invalidate, p, e);

    if (p < e)
        goto more;

    /* invalidate when we reach the _end_ (root) of the trace */
    if (invalidate)
        rebuild_snap_realms(realm);

    /*
     * queue cap snaps _after_ we've built the new snap contexts,
     * so that i_head_snapc can be set appropriately.
     */
    while (!list_empty(&dirty_realms)) {
        realm = list_first_entry(&dirty_realms, struct ceph_snap_realm,
                     dirty_item);
        queue_realm_cap_snaps(realm);
    }

    __cleanup_empty_realms(mdsc);
    return 0;

bad:
    err = -EINVAL;
fail:
    pr_err("update_snap_trace error %d\n", err);
    return err;
}


/*
 * Send any cap_snaps that are queued for flush.  Try to carry
 * s_mutex across multiple snap flushes to avoid locking overhead.
 *
 * Caller holds no locks.
 */
static void flush_snaps(struct ceph_mds_client *mdsc)
{
    struct ceph_inode_info *ci;
    struct inode *inode;
    struct ceph_mds_session *session = NULL;

    dout("flush_snaps\n");
    spin_lock(&mdsc->snap_flush_lock);
    while (!list_empty(&mdsc->snap_flush_list)) {
        ci = list_first_entry(&mdsc->snap_flush_list,
                struct ceph_inode_info, i_snap_flush_item);
        inode = &ci->vfs_inode;
        ihold(inode);
        spin_unlock(&mdsc->snap_flush_lock);
        spin_lock(&ci->i_ceph_lock);
        __ceph_flush_snaps(ci, &session, 0);
        spin_unlock(&ci->i_ceph_lock);
        iput(inode);
        spin_lock(&mdsc->snap_flush_lock);
    }
    spin_unlock(&mdsc->snap_flush_lock);

    if (session) {
        mutex_unlock(&session->s_mutex);
        ceph_put_mds_session(session);
    }
    dout("flush_snaps done\n");
}


/*
 * Handle a snap notification from the MDS.
 *
 * This can take two basic forms: the simplest is just a snap creation
 * or deletion notification on an existing realm.  This should update the
 * realm and its children.
 *
 * The more difficult case is realm creation, due to snap creation at a
 * new point in the file hierarchy, or due to a rename that moves a file or
 * directory into another realm.
 */
void ceph_handle_snap(struct ceph_mds_client *mdsc,
              struct ceph_mds_session *session,
              struct ceph_msg *msg)
{
    struct super_block *sb = mdsc->fsc->sb;
    int mds = session->s_mds;
    u64 split;
    int op;
    int trace_len;
    struct ceph_snap_realm *realm = NULL;
    void *p = msg->front.iov_base;
    void *e = p + msg->front.iov_len;
    struct ceph_mds_snap_head *h;
    int num_split_inos, num_split_realms;
    __le64 *split_inos = NULL, *split_realms = NULL;
    int i;
    int locked_rwsem = 0;

    /* decode */
    if (msg->front.iov_len < sizeof(*h))
        goto bad;
    h = p;
    op = le32_to_cpu(h->op);
    split = le64_to_cpu(h->split);   /* non-zero if we are splitting an
                      * existing realm */
    num_split_inos = le32_to_cpu(h->num_split_inos);
    num_split_realms = le32_to_cpu(h->num_split_realms);
    trace_len = le32_to_cpu(h->trace_len);
    p += sizeof(*h);

    dout("handle_snap from mds%d op %s split %llx tracelen %d\n", mds,
         ceph_snap_op_name(op), split, trace_len);

    mutex_lock(&session->s_mutex);
    session->s_seq++;
    mutex_unlock(&session->s_mutex);

    down_write(&mdsc->snap_rwsem);
    locked_rwsem = 1;

    if (op == CEPH_SNAP_OP_SPLIT) {
        struct ceph_mds_snap_realm *ri;

        /*
         * A "split" breaks part of an existing realm off into
         * a new realm.  The MDS provides a list of inodes
         * (with caps) and child realms that belong to the new
         * child.
         */
        split_inos = p;
        p += sizeof(u64) * num_split_inos;
        split_realms = p;
        p += sizeof(u64) * num_split_realms;
        ceph_decode_need(&p, e, sizeof(*ri), bad);
        /* we will peek at realm info here, but will _not_
         * advance p, as the realm update will occur below in
         * ceph_update_snap_trace. */
        ri = p;

        realm = ceph_lookup_snap_realm(mdsc, split);
        if (!realm) {
            realm = ceph_create_snap_realm(mdsc, split);
            if (IS_ERR(realm))
                goto out;
        }
        ceph_get_snap_realm(mdsc, realm);

        dout("splitting snap_realm %llx %p\n", realm->ino, realm);
        for (i = 0; i < num_split_inos; i++) {
            struct ceph_vino vino = {
                .ino = le64_to_cpu(split_inos[i]),
                .snap = CEPH_NOSNAP,
            };
            struct inode *inode = ceph_find_inode(sb, vino);
            struct ceph_inode_info *ci;
            struct ceph_snap_realm *oldrealm;

            if (!inode)
                continue;
            ci = ceph_inode(inode);

            spin_lock(&ci->i_ceph_lock);
            if (!ci->i_snap_realm)
                goto skip_inode;
            /*
             * If this inode belongs to a realm that was
             * created after our new realm, we experienced
             * a race (due to another split notifications
             * arriving from a different MDS).  So skip
             * this inode.
             */
            if (ci->i_snap_realm->created >
                le64_to_cpu(ri->created)) {
                dout(" leaving %p in newer realm %llx %p\n",
                     inode, ci->i_snap_realm->ino,
                     ci->i_snap_realm);
                goto skip_inode;
            }
            dout(" will move %p to split realm %llx %p\n",
                 inode, realm->ino, realm);
            /*
             * Move the inode to the new realm
             */
            spin_lock(&realm->inodes_with_caps_lock);
            list_del_init(&ci->i_snap_realm_item);
            list_add(&ci->i_snap_realm_item,
                 &realm->inodes_with_caps);
            oldrealm = ci->i_snap_realm;
            ci->i_snap_realm = realm;
            spin_unlock(&realm->inodes_with_caps_lock);
            spin_unlock(&ci->i_ceph_lock);

            ceph_get_snap_realm(mdsc, realm);
            ceph_put_snap_realm(mdsc, oldrealm);

            iput(inode);
            continue;

skip_inode:
            spin_unlock(&ci->i_ceph_lock);
            iput(inode);
        }

        /* we may have taken some of the old realm's children. */
        for (i = 0; i < num_split_realms; i++) {
            struct ceph_snap_realm *child =
                ceph_lookup_snap_realm(mdsc,
                       le64_to_cpu(split_realms[i]));
            if (!child)
                continue;
            adjust_snap_realm_parent(mdsc, child, realm->ino);
        }
    }

    /*
     * update using the provided snap trace. if we are deleting a
     * snap, we can avoid queueing cap_snaps.
     */
    ceph_update_snap_trace(mdsc, p, e,
                   op == CEPH_SNAP_OP_DESTROY);

    if (op == CEPH_SNAP_OP_SPLIT)
        /* we took a reference when we created the realm, above */
        ceph_put_snap_realm(mdsc, realm);

    __cleanup_empty_realms(mdsc);

    up_write(&mdsc->snap_rwsem);

    flush_snaps(mdsc);
    return;

bad:
    pr_err("corrupt snap message from mds%d\n", mds);
    ceph_msg_dump(msg);
out:
    if (locked_rwsem)
        up_write(&mdsc->snap_rwsem);
    return;
}