/*
 * 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/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>

struct backing_dev_info default_backing_dev_info = {
        .ra_pages       = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
        .state          = 0,
};

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

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

static int
read_pages(struct file *file, struct address_space *mapping,
                struct list_head *pages, unsigned nr_pages)
{
        unsigned page_idx;
        struct pagevec lru_pvec;

        pagevec_init(&lru_pvec);

        if (mapping->a_ops->readpages)
                return mapping->a_ops->readpages(mapping, pages, nr_pages);

        for (page_idx = 0; page_idx < nr_pages; page_idx++) {
                struct page *page = list_entry(pages->prev, struct page, list);
                list_del(&page->list);
                if (!add_to_page_cache(page, mapping, page->index)) {
                        mapping->a_ops->readpage(file, page);
                        if (!pagevec_add(&lru_pvec, page))
                                __pagevec_lru_add(&lru_pvec);
                } else {
                        page_cache_release(page);
                }
        }
        pagevec_lru_add(&lru_pvec);
        return 0;
}

/*
 * 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.
 *
 * 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.
 *
 * 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.
 */
int do_page_cache_readahead(struct file *file,
                        unsigned long offset, unsigned long nr_to_read)
{
        struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
        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;

        if (inode->i_size == 0)
                goto out;

        end_index = ((inode->i_size - 1) >> PAGE_CACHE_SHIFT);

        /*
         * Preallocate as many pages as we will need.
         */
        read_lock(&mapping->page_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;

                read_unlock(&mapping->page_lock);
                page = page_cache_alloc(mapping);
                read_lock(&mapping->page_lock);
                if (!page)
                        break;
                page->index = page_offset;
                list_add(&page->list, &page_pool);
                ret++;
        }
        read_unlock(&mapping->page_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(file, mapping, &page_pool, ret);
                blk_run_queues();
        }
        BUG_ON(!list_empty(&page_pool));
out:
        return ret;
}

/*
 * 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;
                }
        }
}

/*
 * page_cache_readahead is the main function.  If performs the adaptive
 * readahead window size management and submits the readahead I/O.
 */
void page_cache_readahead(struct file *file, unsigned long offset)
{
        struct file_ra_state *ra = &file->f_ra;
        unsigned max;
        unsigned min;
        unsigned orig_next_size;
        unsigned actual;

        /*
         * 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;
        }

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

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

        min = get_min_readahead(file);
        orig_next_size = ra->next_size;

        if (ra->next_size == 0 && offset == 0) {
                /*
                 * Special case - first read from first page.
                 * We'll assume it's a whole-file read, and
                 * grow the window fast.
                 */
                ra->next_size = max / 2;
                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;
        } else {
                /*
                 * A miss - lseek, pread, etc.  Shrink the readahead
                 * window by 25%.
                 */
                ra->next_size -= ra->next_size / 4;
        }

        if (ra->next_size > max)
                ra->next_size = max;
        if (ra->next_size < min)
                ra->next_size = min;

        /*
         * 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.
                 */
                ra->start = offset;
                ra->size = ra->next_size;
                ra->ahead_start = 0;            /* Invalidate these */
                ra->ahead_size = 0;

                actual = do_page_cache_readahead(file, offset, ra->size);
                check_ra_success(ra, ra->size, actual, orig_next_size);
        } else {
                /*
                 * This read request is within the current window.  It is time
                 * to submit I/O for the ahead window while the application is
                 * crunching through the current window.
                 */
                if (ra->ahead_start == 0) {
                        ra->ahead_start = ra->start + ra->size;
                        ra->ahead_size = ra->next_size;
                        actual = do_page_cache_readahead(file,
                                        ra->ahead_start, ra->ahead_size);
                        check_ra_success(ra, ra->ahead_size,
                                        actual, orig_next_size);
                }
        }
out:
        return;
}

/*
 * For mmap reads (typically executables) the access pattern is fairly random,
 * but somewhat ascending.  So readaround favours pages beyond the target one.
 * We also boost the window size, as it can easily shrink due to misses.
 */
void page_cache_readaround(struct file *file, unsigned long offset)
{
        struct file_ra_state *ra = &file->f_ra;

        if (ra->next_size != -1UL) {
                const unsigned long min = get_min_readahead(file) * 2;
                unsigned long target;
                unsigned long backward;

                /*
                 * If next_size is zero then leave it alone, because that's a
                 * readahead startup state.
                 */
                if (ra->next_size && ra->next_size < min)
                        ra->next_size = min;

                target = offset;
                backward = ra->next_size / 4;

                if (backward > target)
                        target = 0;
                else
                        target -= backward;
                page_cache_readahead(file, target);
        }
}

/*
 * 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 == 0) then bump it up
 * again to 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 file *file)
{
        struct file_ra_state *ra = &file->f_ra;
        const unsigned long min = get_min_readahead(file);

        if (ra->next_size == -1UL) {
                ra->next_size = min;
        } else {
                ra->next_size -= 3;
                if (ra->next_size < min)
                        ra->next_size = min;
        }
}