readahead.c

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/*
 * mm/readahead.c - address_space-level file readahead.
 *
 * Copyright (C) 2002, Linus Torvalds
 *
 * 09Apr2002    Andrew Morton
 *      Initial version.
 */

#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/gfp.h>
#include <linux/mm.h>
#include <linux/export.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/pagevec.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
#include <linux/file.h>

/*
 * Initialise a struct file's readahead state.  Assumes that the caller has
 * memset *ra to zero.
 */
void
file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
{
    ra->ra_pages = mapping->backing_dev_info->ra_pages;
    ra->prev_pos = -1;
}
EXPORT_SYMBOL_GPL(file_ra_state_init);

#define list_to_page(head) (list_entry((head)->prev, struct page, lru))

/*
 * see if a page needs releasing upon read_cache_pages() failure
 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
 *   before calling, such as the NFS fs marking pages that are cached locally
 *   on disk, thus we need to give the fs a chance to clean up in the event of
 *   an error
 */
static void read_cache_pages_invalidate_page(struct address_space *mapping,
                         struct page *page)
{
    if (page_has_private(page)) {
        if (!trylock_page(page))
            BUG();
        page->mapping = mapping;
        do_invalidatepage(page, 0);
        page->mapping = NULL;
        unlock_page(page);
    }
    page_cache_release(page);
}

/*
 * release a list of pages, invalidating them first if need be
 */
static void read_cache_pages_invalidate_pages(struct address_space *mapping,
                          struct list_head *pages)
{
    struct page *victim;

    while (!list_empty(pages)) {
        victim = list_to_page(pages);
        list_del(&victim->lru);
        read_cache_pages_invalidate_page(mapping, victim);
    }
}

int read_cache_pages(struct address_space *mapping, struct list_head *pages,
            int (*filler)(void *, struct page *), void *data)
{
    struct page *page;
    int ret = 0;

    while (!list_empty(pages)) {
        page = list_to_page(pages);
        list_del(&page->lru);
        if (add_to_page_cache_lru(page, mapping,
                    page->index, GFP_KERNEL)) {
            read_cache_pages_invalidate_page(mapping, page);
            continue;
        }
        page_cache_release(page);

        ret = filler(data, page);
        if (unlikely(ret)) {
            read_cache_pages_invalidate_pages(mapping, pages);
            break;
        }
        task_io_account_read(PAGE_CACHE_SIZE);
    }
    return ret;
}

EXPORT_SYMBOL(read_cache_pages);

static int read_pages(struct address_space *mapping, struct file *filp,
        struct list_head *pages, unsigned nr_pages)
{
    struct blk_plug plug;
    unsigned page_idx;
    int ret;

    blk_start_plug(&plug);

    if (mapping->a_ops->readpages) {
        ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
        /* Clean up the remaining pages */
        put_pages_list(pages);
        goto out;
    }

    for (page_idx = 0; page_idx < nr_pages; page_idx++) {
        struct page *page = list_to_page(pages);
        list_del(&page->lru);
        if (!add_to_page_cache_lru(page, mapping,
                    page->index, GFP_KERNEL)) {
            mapping->a_ops->readpage(filp, page);
        }
        page_cache_release(page);
    }
    ret = 0;

out:
    blk_finish_plug(&plug);

    return ret;
}

/*
 * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
 * the pages first, then submits them all for I/O. This avoids the very bad
 * behaviour which would occur if page allocations are causing VM writeback.
 * We really don't want to intermingle reads and writes like that.
 *
 * Returns the number of pages requested, or the maximum amount of I/O allowed.
 */
static int
__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
            pgoff_t offset, unsigned long nr_to_read,
            unsigned long lookahead_size)
{
    struct inode *inode = mapping->host;
    struct page *page;
    unsigned long end_index;    /* The last page we want to read */
    LIST_HEAD(page_pool);
    int page_idx;
    int ret = 0;
    loff_t isize = i_size_read(inode);

    if (isize == 0)
        goto out;

    end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);

    /*
     * Preallocate as many pages as we will need.
     */
    for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
        pgoff_t page_offset = offset + page_idx;

        if (page_offset > end_index)
            break;

        rcu_read_lock();
        page = radix_tree_lookup(&mapping->page_tree, page_offset);
        rcu_read_unlock();
        if (page)
            continue;

        page = page_cache_alloc_readahead(mapping);
        if (!page)
            break;
        page->index = page_offset;
        list_add(&page->lru, &page_pool);
        if (page_idx == nr_to_read - lookahead_size)
            SetPageReadahead(page);
        ret++;
    }

    /*
     * Now start the IO.  We ignore I/O errors - if the page is not
     * uptodate then the caller will launch readpage again, and
     * will then handle the error.
     */
    if (ret)
        read_pages(mapping, filp, &page_pool, ret);
    BUG_ON(!list_empty(&page_pool));
out:
    return ret;
}

/*
 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 * memory at once.
 */
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
        pgoff_t offset, unsigned long nr_to_read)
{
    int ret = 0;

    if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
        return -EINVAL;

    nr_to_read = max_sane_readahead(nr_to_read);
    while (nr_to_read) {
        int err;

        unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;

        if (this_chunk > nr_to_read)
            this_chunk = nr_to_read;
        err = __do_page_cache_readahead(mapping, filp,
                        offset, this_chunk, 0);
        if (err < 0) {
            ret = err;
            break;
        }
        ret += err;
        offset += this_chunk;
        nr_to_read -= this_chunk;
    }
    return ret;
}

/*
 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
 * sensible upper limit.
 */
unsigned long max_sane_readahead(unsigned long nr)
{
    return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
        + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
}

/*
 * Submit IO for the read-ahead request in file_ra_state.
 */
unsigned long ra_submit(struct file_ra_state *ra,
               struct address_space *mapping, struct file *filp)
{
    int actual;

    actual = __do_page_cache_readahead(mapping, filp,
                    ra->start, ra->size, ra->async_size);

    return actual;
}

/*
 * Set the initial window size, round to next power of 2 and square
 * for small size, x 4 for medium, and x 2 for large
 * for 128k (32 page) max ra
 * 1-8 page = 32k initial, > 8 page = 128k initial
 */
static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
{
    unsigned long newsize = roundup_pow_of_two(size);

    if (newsize <= max / 32)
        newsize = newsize * 4;
    else if (newsize <= max / 4)
        newsize = newsize * 2;
    else
        newsize = max;

    return newsize;
}

/*
 *  Get the previous window size, ramp it up, and
 *  return it as the new window size.
 */
static unsigned long get_next_ra_size(struct file_ra_state *ra,
                        unsigned long max)
{
    unsigned long cur = ra->size;
    unsigned long newsize;

    if (cur < max / 16)
        newsize = 4 * cur;
    else
        newsize = 2 * cur;

    return min(newsize, max);
}

/*
 * On-demand readahead design.
 *
 * The fields in struct file_ra_state represent the most-recently-executed
 * readahead attempt:
 *
 *                        |<----- async_size ---------|
 *     |------------------- size -------------------->|
 *     |==================#===========================|
 *     ^start             ^page marked with PG_readahead
 *
 * To overlap application thinking time and disk I/O time, we do
 * `readahead pipelining': Do not wait until the application consumed all
 * readahead pages and stalled on the missing page at readahead_index;
 * Instead, submit an asynchronous readahead I/O as soon as there are
 * only async_size pages left in the readahead window. Normally async_size
 * will be equal to size, for maximum pipelining.
 *
 * In interleaved sequential reads, concurrent streams on the same fd can
 * be invalidating each other's readahead state. So we flag the new readahead
 * page at (start+size-async_size) with PG_readahead, and use it as readahead
 * indicator. The flag won't be set on already cached pages, to avoid the
 * readahead-for-nothing fuss, saving pointless page cache lookups.
 *
 * prev_pos tracks the last visited byte in the _previous_ read request.
 * It should be maintained by the caller, and will be used for detecting
 * small random reads. Note that the readahead algorithm checks loosely
 * for sequential patterns. Hence interleaved reads might be served as
 * sequential ones.
 *
 * There is a special-case: if the first page which the application tries to
 * read happens to be the first page of the file, it is assumed that a linear
 * read is about to happen and the window is immediately set to the initial size
 * based on I/O request size and the max_readahead.
 *
 * The code ramps up the readahead size aggressively at first, but slow down as
 * it approaches max_readhead.
 */

/*
 * Count contiguously cached pages from @offset-1 to @offset-@max,
 * this count is a conservative estimation of
 *  - length of the sequential read sequence, or
 *  - thrashing threshold in memory tight systems
 */
static pgoff_t count_history_pages(struct address_space *mapping,
                   struct file_ra_state *ra,
                   pgoff_t offset, unsigned long max)
{
    pgoff_t head;

    rcu_read_lock();
    head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
    rcu_read_unlock();

    return offset - 1 - head;
}

/*
 * page cache context based read-ahead
 */
static int try_context_readahead(struct address_space *mapping,
                 struct file_ra_state *ra,
                 pgoff_t offset,
                 unsigned long req_size,
                 unsigned long max)
{
    pgoff_t size;

    size = count_history_pages(mapping, ra, offset, max);

    /*
     * no history pages:
     * it could be a random read
     */
    if (!size)
        return 0;

    /*
     * starts from beginning of file:
     * it is a strong indication of long-run stream (or whole-file-read)
     */
    if (size >= offset)
        size *= 2;

    ra->start = offset;
    ra->size = get_init_ra_size(size + req_size, max);
    ra->async_size = ra->size;

    return 1;
}

/*
 * A minimal readahead algorithm for trivial sequential/random reads.
 */
static unsigned long
ondemand_readahead(struct address_space *mapping,
           struct file_ra_state *ra, struct file *filp,
           bool hit_readahead_marker, pgoff_t offset,
           unsigned long req_size)
{
    unsigned long max = max_sane_readahead(ra->ra_pages);

    /*
     * start of file
     */
    if (!offset)
        goto initial_readahead;

    /*
     * It's the expected callback offset, assume sequential access.
     * Ramp up sizes, and push forward the readahead window.
     */
    if ((offset == (ra->start + ra->size - ra->async_size) ||
         offset == (ra->start + ra->size))) {
        ra->start += ra->size;
        ra->size = get_next_ra_size(ra, max);
        ra->async_size = ra->size;
        goto readit;
    }

    /*
     * Hit a marked page without valid readahead state.
     * E.g. interleaved reads.
     * Query the pagecache for async_size, which normally equals to
     * readahead size. Ramp it up and use it as the new readahead size.
     */
    if (hit_readahead_marker) {
        pgoff_t start;

        rcu_read_lock();
        start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
        rcu_read_unlock();

        if (!start || start - offset > max)
            return 0;

        ra->start = start;
        ra->size = start - offset;  /* old async_size */
        ra->size += req_size;
        ra->size = get_next_ra_size(ra, max);
        ra->async_size = ra->size;
        goto readit;
    }

    /*
     * oversize read
     */
    if (req_size > max)
        goto initial_readahead;

    /*
     * sequential cache miss
     */
    if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
        goto initial_readahead;

    /*
     * Query the page cache and look for the traces(cached history pages)
     * that a sequential stream would leave behind.
     */
    if (try_context_readahead(mapping, ra, offset, req_size, max))
        goto readit;

    /*
     * standalone, small random read
     * Read as is, and do not pollute the readahead state.
     */
    return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);

initial_readahead:
    ra->start = offset;
    ra->size = get_init_ra_size(req_size, max);
    ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;

readit:
    /*
     * Will this read hit the readahead marker made by itself?
     * If so, trigger the readahead marker hit now, and merge
     * the resulted next readahead window into the current one.
     */
    if (offset == ra->start && ra->size == ra->async_size) {
        ra->async_size = get_next_ra_size(ra, max);
        ra->size += ra->async_size;
    }

    return ra_submit(ra, mapping, filp);
}

void page_cache_sync_readahead(struct address_space *mapping,
                   struct file_ra_state *ra, struct file *filp,
                   pgoff_t offset, unsigned long req_size)
{
    /* no read-ahead */
    if (!ra->ra_pages)
        return;

    /* be dumb */
    if (filp && (filp->f_mode & FMODE_RANDOM)) {
        force_page_cache_readahead(mapping, filp, offset, req_size);
        return;
    }

    /* do read-ahead */
    ondemand_readahead(mapping, ra, filp, false, offset, req_size);
}
EXPORT_SYMBOL_GPL(page_cache_sync_readahead);

void
page_cache_async_readahead(struct address_space *mapping,
               struct file_ra_state *ra, struct file *filp,
               struct page *page, pgoff_t offset,
               unsigned long req_size)
{
    /* no read-ahead */
    if (!ra->ra_pages)
        return;

    /*
     * Same bit is used for PG_readahead and PG_reclaim.
     */
    if (PageWriteback(page))
        return;

    ClearPageReadahead(page);

    /*
     * Defer asynchronous read-ahead on IO congestion.
     */
    if (bdi_read_congested(mapping->backing_dev_info))
        return;

    /* do read-ahead */
    ondemand_readahead(mapping, ra, filp, true, offset, req_size);
}
EXPORT_SYMBOL_GPL(page_cache_async_readahead);

static ssize_t
do_readahead(struct address_space *mapping, struct file *filp,
         pgoff_t index, unsigned long nr)
{
    if (!mapping || !mapping->a_ops || !mapping->a_ops->readpage)
        return -EINVAL;

    force_page_cache_readahead(mapping, filp, index, nr);
    return 0;
}

SYSCALL_DEFINE(readahead)(int fd, loff_t offset, size_t count)
{
    ssize_t ret;
    struct fd f;

    ret = -EBADF;
    f = fdget(fd);
    if (f.file) {
        if (f.file->f_mode & FMODE_READ) {
            struct address_space *mapping = f.file->f_mapping;
            pgoff_t start = offset >> PAGE_CACHE_SHIFT;
            pgoff_t end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
            unsigned long len = end - start + 1;
            ret = do_readahead(mapping, f.file, start, len);
        }
        fdput(f);
    }
    return ret;
}
#ifdef CONFIG_HAVE_SYSCALL_WRAPPERS
asmlinkage long SyS_readahead(long fd, loff_t offset, long count)
{
    return SYSC_readahead((int) fd, offset, (size_t) count);
}
SYSCALL_ALIAS(sys_readahead, SyS_readahead);
#endif