/*
 * 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>

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,
        .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->average = ra->ra_pages / 2;
}

/*
 * Return max readahead size for this inode in number-of-pages.
 */
static inline unsigned long get_max_readahead(struct file_ra_state *ra)
{
        return ra->ra_pages;
}

static inline unsigned long get_min_readahead(struct file_ra_state *ra)
{
        return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
}

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

/**
 * read_cache_pages - populate an address space with some pages, and
 *                      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;
        struct pagevec lru_pvec;
        int ret = 0;

        pagevec_init(&lru_pvec, 0);

        while (!list_empty(pages)) {
                page = list_to_page(pages);
                list_del(&page->lru);
                if (add_to_page_cache(page, mapping, page->index, GFP_KERNEL)) {
                        page_cache_release(page);
                        continue;
                }
                ret = filler(data, page);
                if (!pagevec_add(&lru_pvec, page))
                        __pagevec_lru_add(&lru_pvec);
                if (ret) {
                        while (!list_empty(pages)) {
                                struct page *victim;

                                victim = list_to_page(pages);
                                list_del(&victim->lru);
                                page_cache_release(victim);
                        }
                        break;
                }
                task_io_account_read(PAGE_CACHE_SIZE);
        }
        pagevec_lru_add(&lru_pvec);
        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;
        struct pagevec lru_pvec;
        int ret = 0;

        if (mapping->a_ops->readpages) {
                ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
                goto out;
        }

        pagevec_init(&lru_pvec, 0);
        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(page, mapping,
                                        page->index, GFP_KERNEL)) {
                        mapping->a_ops->readpage(filp, page);
                        if (!pagevec_add(&lru_pvec, page))
                                __pagevec_lru_add(&lru_pvec);
                } else {
                        page_cache_release(page);
                }
        }
        pagevec_lru_add(&lru_pvec);
out:
        return ret;
}

/*
 * Readahead design.
 *
 * The fields in struct file_ra_state represent the most-recently-executed
 * readahead attempt:
 *
 * start:       Page index at which we started the readahead
 * size:        Number of pages in that read
 *              Together, these form the "current window".
 *              Together, start and size represent the `readahead window'.
 * next_size:   The number of pages to read on the next readahead miss.
 *              Has the magical value -1UL if readahead has been disabled.
 * prev_page:   The page which the readahead algorithm most-recently inspected.
 *              prev_page is mainly an optimisation: if page_cache_readahead
 *              sees that it is again being called for a page which it just
 *              looked at, it can return immediately without making any state
 *              changes.
 * ahead_start,
 * ahead_size:  Together, these form the "ahead window".
 * ra_pages:    The externally controlled max readahead for this fd.
 *
 * When readahead is in the "maximally shrunk" state (next_size == -1UL),
 * readahead is disabled.  In this state, prev_page and size are used, inside
 * handle_ra_miss(), to detect the resumption of sequential I/O.  Once there
 * has been a decent run of sequential I/O (defined by get_min_readahead),
 * readahead is reenabled.
 *
 * The readahead code manages two windows - the "current" and the "ahead"
 * windows.  The intent is that while the application is walking the pages
 * in the current window, I/O is underway on the ahead window.  When the
 * current window is fully traversed, it is replaced by the ahead window
 * and the ahead window is invalidated.  When this copying happens, the
 * new current window's pages are probably still locked.  When I/O has
 * completed, we submit a new batch of I/O, creating a new ahead window.
 *
 * So:
 *
 *   ----|----------------|----------------|-----
 *       ^start           ^start+size
 *                        ^ahead_start     ^ahead_start+ahead_size
 *
 *         ^ When this page is read, we submit I/O for the
 *           ahead window.
 *
 * A `readahead hit' occurs when a read request is made against a page which is
 * inside the current window.  Hits are good, and the window size (next_size)
 * is grown aggressively when hits occur.  Two pages are added to the next
 * window size on each hit, which will end up doubling the next window size by
 * the time I/O is submitted for it.
 *
 * If readahead hits are more sparse (say, the application is only reading
 * every second page) then the window will build more slowly.
 *
 * On a readahead miss (the application seeked away) the readahead window is
 * shrunk by 25%.  We don't want to drop it too aggressively, because it is a
 * good assumption that an application which has built a good readahead window
 * will continue to perform linear reads.  Either at the new file position, or
 * at the old one after another seek.
 *
 * After enough misses, readahead is fully disabled. (next_size = -1UL).
 *
 * 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 half of the
 * device maximum.
 * 
 * A page request at (start + size) is not a miss at all - it's just a part of
 * sequential file reading.
 *
 * This function is to be called for every page which is read, rather than when
 * it is time to perform readahead.  This is so the readahead algorithm can
 * centrally work out the access patterns.  This could be costly with many tiny
 * read()s, so we specifically optimise for that case with prev_page.
 */

/*
 * 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 which actually had IO started against them.
 */
static inline int
__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
                        unsigned long offset, unsigned long nr_to_read)
{
        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.
         */
        spin_lock_irq(&mapping->tree_lock);
        for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
                unsigned long page_offset = offset + page_idx;
                
                if (page_offset > end_index)
                        break;

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

                spin_unlock_irq(&mapping->tree_lock);
                page = page_cache_alloc_cold(mapping);
                spin_lock_irq(&mapping->tree_lock);
                if (!page)
                        break;
                page->index = page_offset;
                list_add(&page->lru, &page_pool);
                ret++;
        }
        spin_unlock_irq(&mapping->tree_lock);

        /*
         * 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,
                unsigned long 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);
                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,
                        unsigned long offset, unsigned long nr_to_read)
{
        if (!bdi_read_congested(mapping->backing_dev_info))
                return __do_page_cache_readahead(mapping, filp,
                                                offset, nr_to_read);
        return 0;
}

/*
 * Check how effective readahead is being.  If the amount of started IO is
 * less than expected then the file is partly or fully in pagecache and
 * readahead isn't helping.  Shrink the window.
 *
 * But don't shrink it too much - the application may read the same page
 * occasionally.
 */
static inline void
check_ra_success(struct file_ra_state *ra, pgoff_t attempt,
                        pgoff_t actual, pgoff_t orig_next_size)
{
        if (actual == 0) {
                if (orig_next_size > 1) {
                        ra->next_size = orig_next_size - 1;
                        if (ra->ahead_size)
                                ra->ahead_size = ra->next_size;
                } else {
                        ra->next_size = -1UL;
                        ra->size = 0;
                }
        }
}

/*
 * Move towards, or away from, re-enabling readahead.
 *
 * For sequential IOs, move ra->size towards re-enabling
 * readahead by the number of sequential pages read.
 * Non-sequential IOs will move away from re-enabling
 * readahead, by decreasing ra->size.
 */

static inline void
check_ra_reenable(struct file_ra_state *ra, unsigned long offset,
                  const unsigned long max, unsigned long nr_read)
{
        if (offset != ra->prev_page + 1) {      /* Not sequential */
                ra->size = ra->size?ra->size-1:0;
        } else {                                /* A sequential read */
                ra->size += nr_read;
                if (ra->size >= max) {          /* Resume readahead */
                        ra->start = offset - max;
                        ra->next_size = max;
                        ra->size = max;
                        ra->ahead_start = 0;
                        ra->ahead_size = 0;
                        ra->average = max / 2;
                }
        }
}

/*
 * page_cache_readahead is the main function.  If performs the adaptive
 * readahead window size management and submits the readahead I/O.
 */
unsigned long
page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
                     struct file *filp, unsigned long offset,
                     unsigned long nr_to_read)
{
        unsigned max;
        unsigned orig_next_size;
        unsigned actual;
        int first_access=0;
        unsigned long average;

        /*
         * Here we detect the case where the application is performing
         * sub-page sized reads.  We avoid doing extra work and bogusly
         * perturbing the readahead window expansion logic.
         * If next_size is zero, this is the very first read for this
         * file handle, or the window is maximally shrunk.
         */
        if (offset == ra->prev_page) {
                if (ra->next_size != 0)
                        goto out;
        }

        max = get_max_readahead(ra);
        if (max == 0)
                goto out;       /* No readahead */

        if (ra->next_size == -1UL)
                goto ra_off;    /* Maximally shrunk */

        orig_next_size = ra->next_size;

        if (ra->next_size == 0) {
                /*
                 * Special case - first read.
                 * We'll assume it's a whole-file read, and
                 * grow the window fast.
                 */
                first_access=1;
                ra->next_size = max / 2;
                ra->prev_page = offset;
                ra->currnt_wnd_hit++;
                goto do_io;
        }

        ra->prev_page = offset;

        if (offset >= ra->start && offset <= (ra->start + ra->size)) {
                /*
                 * A readahead hit.  Either inside the window, or one
                 * page beyond the end.  Expand the next readahead size.
                 */
                ra->next_size += 2;

                if (ra->currnt_wnd_hit <= (max * 2))
                        ra->currnt_wnd_hit++;
        } else {
                /*
                 * A miss - lseek, pagefault, pread, etc.  Shrink the readahead
                 * window.
                 */
                ra->next_size -= 2;

                average = ra->average;
                if (average < ra->currnt_wnd_hit) {
                        average++;
                }
                ra->average = (average + ra->currnt_wnd_hit) / 2;
                ra->currnt_wnd_hit = 1;
        }

        if ((long)ra->next_size > (long)max)
                ra->next_size = max;
        if ((long)ra->next_size <= 0L) {
                ra->next_size = -1UL;
                ra->size = 0;
                goto ra_off;            /* Readahead is off */
        }

        /*
         * Is this request outside the current window?
         */
        if (offset < ra->start || offset >= (ra->start + ra->size)) {
                /*
                 * A miss against the current window.  Have we merely
                 * advanced into the ahead window?
                 */
                if (offset == ra->ahead_start) {
                        /*
                         * Yes, we have.  The ahead window now becomes
                         * the current window.
                         */
                        ra->start = ra->ahead_start;
                        ra->size = ra->ahead_size;
                        ra->prev_page = ra->start;
                        ra->ahead_start = 0;
                        ra->ahead_size = 0;

                        /*
                         * Control now returns, probably to sleep until I/O
                         * completes against the first ahead page.
                         * When the second page in the old ahead window is
                         * requested, control will return here and more I/O
                         * will be submitted to build the new ahead window.
                         */
                        goto out;
                }
do_io:
                /*
                 * This is the "unusual" path.  We come here during
                 * startup or after an lseek.  We invalidate the
                 * ahead window and get some I/O underway for the new
                 * current window.
                 */
                if (!first_access) {
                         /* Heuristic: there is a high probability
                          * that around  ra->average number of
                          * pages shall be accessed in the next
                          * current window.
                          */
                        average = ra->average;
                        if (ra->currnt_wnd_hit > average)
                                average = (ra->currnt_wnd_hit + ra->average + 1) / 2;

                        ra->next_size = min(average , (unsigned long)max);
                }
                ra->start = offset;
                ra->size = ra->next_size;
                if (ra->size < nr_to_read)
                        ra->size = min(nr_to_read, (unsigned long)max);
                ra->prev_page = offset + ra->size - 1;
                ra->ahead_start = 0;            /* Invalidate these */
                ra->ahead_size = 0;
                actual = do_page_cache_readahead(mapping, filp, offset,
                                                 ra->size);
                if(!first_access) {
                        /*
                         * do not adjust the readahead window size the first
                         * time, the ahead window might get closed if all
                         * the pages are already in the cache.
                         */
                        check_ra_success(ra, ra->size, actual, orig_next_size);
                }
        } else {
                /*
                 * This read request is within the current window.  It may be
                 * time to submit I/O for the ahead window while the
                 * application is about to step into the ahead window.
                 */
                if (ra->ahead_start == 0) {
                        /*
                         * If the average io-size is more than maximum
                         * readahead size of the file the io pattern is
                         * sequential. Hence  bring in the readahead window
                         * immediately.
                         * If the average io-size is less than maximum
                         * readahead size of the file the io pattern is
                         * random. Hence don't bother to readahead.
                         */
                        average = ra->average;
                        if (ra->currnt_wnd_hit > average)
                                average = (ra->currnt_wnd_hit + ra->average + 1) / 2;

                        if (average > max) {
                                ra->ahead_start = ra->start + ra->size;
                                ra->ahead_size = ra->next_size;
                                actual = do_page_cache_readahead(mapping, filp,
                                        ra->ahead_start, ra->ahead_size);
                                check_ra_success(ra, ra->ahead_size,
                                                actual, orig_next_size);
                        }
                }
        }
out:
        return ra->prev_page + 1;

ra_off:
        /* Even if we have decided to terminate readahead for this file,
         * a single large read request still wants to be able to
         * populate the page cache with a single large bio, so if
         * necessary, kick off one limited readahead IO burst to
         * complete as much of the read as we can.
         */
        if (nr_to_read > 1) {
                nr_to_read = min(nr_to_read, (unsigned long) max);
                check_ra_reenable(ra, offset, max, nr_to_read);
                ra->prev_page = offset + nr_to_read - 1;
                do_page_cache_readahead(mapping, filp, offset, nr_to_read);
        }
        goto out;
}


/*
 * handle_ra_miss() is called when it is known that a page which should have
 * been present in the pagecache (we just did some readahead there) was in fact
 * not found.  This will happen if it was evicted by the VM (readahead
 * thrashing) or if the readahead window is maximally shrunk.
 *
 * If the window has been maximally shrunk (next_size == -1UL) then look to see
 * if we are getting misses against sequential file offsets.  If so, and this
 * persists then resume readahead.
 *
 * Otherwise we're thrashing, so shrink the readahead window by three pages.
 * This is because it is grown by two pages on a readahead hit.  Theory being
 * that the readahead window size will stabilise around the maximum level at
 * which there is no thrashing.
 */
void handle_ra_miss(struct address_space *mapping,
                struct file_ra_state *ra, pgoff_t offset)
{
        if (ra->next_size == -1UL) {
                const unsigned long max = get_max_readahead(ra);

                check_ra_reenable(ra, offset, max, 1);
                ra->prev_page = offset;
        } else {
                const unsigned long min = get_min_readahead(ra);

                ra->next_size -= 3;
                if (ra->next_size < min)
                        ra->next_size = min;
        }
}

/*
 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
 * sensible upper limit.
 */
unsigned long max_sane_readahead(unsigned long nr)
{
        unsigned long active;
        unsigned long inactive;
        unsigned long free;

        __get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id()));
        return min(nr, (inactive + free) / 2);
}