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/*

 * mm/readahead.c - address_space-level file readahead.

 *

 * Copyright (C) 2002, Linus Torvalds

 *

 * 09Apr2002    akpm@zip.com.au

 *              Initial version.

 */

#include <linux/kernel.h>

#include <linux/fs.h>

#include <linux/mm.h>

#include <linux/module.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>

void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)

{

}

EXPORT_SYMBOL(default_unplug_io_fn);

struct backing_dev_info default_backing_dev_info = {

        .ra_pages       = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,

        .state          = 0,

        .capabilities   = BDI_CAP_MAP_COPY,

        .unplug_io_fn   = default_unplug_io_fn,

};

EXPORT_SYMBOL_GPL(default_backing_dev_info);

/*

 * 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))

/**

 * read_cache_pages - populate an address space with some pages & start reads against them

 * @mapping: the address_space

 * @pages: The address of a list_head which contains the target pages.  These

 *   pages have their ->index populated and are otherwise uninitialised.

 * @filler: callback routine for filling a single page.

 * @data: private data for the callback routine.

 *

 * Hides the details of the LRU cache etc from the filesystems.

 */

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)) {

                        page_cache_release(page);

                        continue;

                }

                page_cache_release(page);

                ret = filler(data, page);

                if (unlikely(ret)) {

                        put_pages_list(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)

{

        unsigned page_idx;

        int ret;

        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:

        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.

 *

 * do_page_cache_readahead() returns -1 if it encountered request queue

 * congestion.

 */

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_cold(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;

        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;

}

/*

 * This version skips the IO if the queue is read-congested, and will tell the

 * block layer to abandon the readahead if request allocation would block.

 *

 * force_page_cache_readahead() will ignore queue congestion and will block on

 * request queues.

 */

int do_page_cache_readahead(struct address_space *mapping, struct file *filp,

                        pgoff_t offset, unsigned long nr_to_read)

{

        if (bdi_read_congested(mapping->backing_dev_info))

                return -1;

        return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);

}

/*

 * 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)

                + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);

}

static int __init readahead_init(void)

{

        return bdi_init(&default_backing_dev_info);

}

subsys_initcall(readahead_init);

/*

 * Submit IO for the read-ahead request in file_ra_state.

 */

static 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.

 */

/*

 * 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)

{

        int     max = ra->ra_pages;     /* max readahead pages */

        pgoff_t prev_offset;

        int     sequential;

        /*

         * It's the expected callback offset, assume sequential access.

         * Ramp up sizes, and push forward the readahead window.

         */

        if (offset && (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;

        }

        prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;

        sequential = offset - prev_offset <= 1UL || req_size > max;

        /*

         * Standalone, small read.

         * Read as is, and do not pollute the readahead state.

         */

        if (!hit_readahead_marker && !sequential) {

                return __do_page_cache_readahead(mapping, filp,

                                                offset, req_size, 0);

        }

        /*

         * 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;

                read_lock_irq(&mapping->tree_lock);

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

                read_unlock_irq(&mapping->tree_lock);

                if (!start || start - offset > max)

                        return 0;

                ra->start = start;

                ra->size = start - offset;      /* old async_size */

                ra->size = get_next_ra_size(ra, max);

                ra->async_size = ra->size;

                goto readit;

        }

        /*

         * It may be one of

         *      - first read on start of file

         *      - sequential cache miss

         *      - oversize random read

         * Start readahead for it.

         */

        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:

        return ra_submit(ra, mapping, filp);

}

/**

 * page_cache_sync_readahead - generic file readahead

 * @mapping: address_space which holds the pagecache and I/O vectors

 * @ra: file_ra_state which holds the readahead state

 * @filp: passed on to ->readpage() and ->readpages()

 * @offset: start offset into @mapping, in pagecache page-sized units

 * @req_size: hint: total size of the read which the caller is performing in

 *            pagecache pages

 *

 * page_cache_sync_readahead() should be called when a cache miss happened:

 * it will submit the read.  The readahead logic may decide to piggyback more

 * pages onto the read request if access patterns suggest it will improve

 * performance.

 */

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;

        /* do read-ahead */

        ondemand_readahead(mapping, ra, filp, false, offset, req_size);

}

EXPORT_SYMBOL_GPL(page_cache_sync_readahead);

/**

 * page_cache_async_readahead - file readahead for marked pages

 * @mapping: address_space which holds the pagecache and I/O vectors

 * @ra: file_ra_state which holds the readahead state

 * @filp: passed on to ->readpage() and ->readpages()

 * @page: the page at @offset which has the PG_readahead flag set

 * @offset: start offset into @mapping, in pagecache page-sized units

 * @req_size: hint: total size of the read which the caller is performing in

 *            pagecache pages

 *

 * page_cache_async_ondemand() should be called when a page is used which

 * has the PG_readahead flag: this is a marker to suggest that the application

 * has used up enough of the readahead window that we should start pulling in

 * more pages. */

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);