// SPDX-License-Identifier: GPL-2.0-only
/*
 * mm/readahead.c - address_space-level file readahead.
 *
 * Copyright (C) 2002, Linus Torvalds
 *
 * 09Apr2002    Andrew Morton
 *              Initial version.
 */

#include <linux/kernel.h>
#include <linux/dax.h>
#include <linux/gfp.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>
#include <linux/mm_inline.h>
#include <linux/blk-cgroup.h>
#include <linux/fadvise.h>
#include <linux/sched/mm.h>

#include "internal.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 = inode_to_bdi(mapping->host)->ra_pages;
        ra->prev_pos = -1;
}
EXPORT_SYMBOL_GPL(file_ra_state_init);

/*
 * 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_SIZE);
                page->mapping = NULL;
                unlock_page(page);
        }
        put_page(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 = lru_to_page(pages);
                list_del(&victim->lru);
                read_cache_pages_invalidate_page(mapping, victim);
        }
}

/**
 * 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.
 *
 * Returns: %0 on success, error return by @filler otherwise
 */
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 = lru_to_page(pages);
                list_del(&page->lru);
                if (add_to_page_cache_lru(page, mapping, page->index,
                                readahead_gfp_mask(mapping))) {
                        read_cache_pages_invalidate_page(mapping, page);
                        continue;
                }
                put_page(page);

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

EXPORT_SYMBOL(read_cache_pages);

static void read_pages(struct readahead_control *rac, struct list_head *pages,
                bool skip_page)
{
        const struct address_space_operations *aops = rac->mapping->a_ops;
        struct page *page;
        struct blk_plug plug;

        if (!readahead_count(rac))
                goto out;

        blk_start_plug(&plug);

        if (aops->readahead) {
                aops->readahead(rac);
                /* Clean up the remaining pages */
                while ((page = readahead_page(rac))) {
                        unlock_page(page);
                        put_page(page);
                }
        } else if (aops->readpages) {
                aops->readpages(rac->file, rac->mapping, pages,
                                readahead_count(rac));
                /* Clean up the remaining pages */
                put_pages_list(pages);
                rac->_index += rac->_nr_pages;
                rac->_nr_pages = 0;
        } else {
                while ((page = readahead_page(rac))) {
                        aops->readpage(rac->file, page);
                        put_page(page);
                }
        }

        blk_finish_plug(&plug);

        BUG_ON(!list_empty(pages));
        BUG_ON(readahead_count(rac));

out:
        if (skip_page)
                rac->_index++;
}

/**
 * page_cache_readahead_unbounded - Start unchecked readahead.
 * @mapping: File address space.
 * @file: This instance of the open file; used for authentication.
 * @index: First page index to read.
 * @nr_to_read: The number of pages to read.
 * @lookahead_size: Where to start the next readahead.
 *
 * This function is for filesystems to call when they want to start
 * readahead beyond a file's stated i_size.  This is almost certainly
 * not the function you want to call.  Use page_cache_async_readahead()
 * or page_cache_sync_readahead() instead.
 *
 * Context: File is referenced by caller.  Mutexes may be held by caller.
 * May sleep, but will not reenter filesystem to reclaim memory.
 */
void page_cache_readahead_unbounded(struct address_space *mapping,
                struct file *file, pgoff_t index, unsigned long nr_to_read,
                unsigned long lookahead_size)
{
        LIST_HEAD(page_pool);
        gfp_t gfp_mask = readahead_gfp_mask(mapping);
        struct readahead_control rac = {
                .mapping = mapping,
                .file = file,
                ._index = index,
        };
        unsigned long i;

        /*
         * Partway through the readahead operation, we will have added
         * locked pages to the page cache, but will not yet have submitted
         * them for I/O.  Adding another page may need to allocate memory,
         * which can trigger memory reclaim.  Telling the VM we're in
         * the middle of a filesystem operation will cause it to not
         * touch file-backed pages, preventing a deadlock.  Most (all?)
         * filesystems already specify __GFP_NOFS in their mapping's
         * gfp_mask, but let's be explicit here.
         */
        unsigned int nofs = memalloc_nofs_save();

        /*
         * Preallocate as many pages as we will need.
         */
        for (i = 0; i < nr_to_read; i++) {
                struct page *page = xa_load(&mapping->i_pages, index + i);

                BUG_ON(index + i != rac._index + rac._nr_pages);

                if (page && !xa_is_value(page)) {
                        /*
                         * Page already present?  Kick off the current batch
                         * of contiguous pages before continuing with the
                         * next batch.  This page may be the one we would
                         * have intended to mark as Readahead, but we don't
                         * have a stable reference to this page, and it's
                         * not worth getting one just for that.
                         */
                        read_pages(&rac, &page_pool, true);
                        continue;
                }

                page = __page_cache_alloc(gfp_mask);
                if (!page)
                        break;
                if (mapping->a_ops->readpages) {
                        page->index = index + i;
                        list_add(&page->lru, &page_pool);
                } else if (add_to_page_cache_lru(page, mapping, index + i,
                                        gfp_mask) < 0) {
                        put_page(page);
                        read_pages(&rac, &page_pool, true);
                        continue;
                }
                if (i == nr_to_read - lookahead_size)
                        SetPageReadahead(page);
                rac._nr_pages++;
        }

        /*
         * 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.
         */
        read_pages(&rac, &page_pool, false);
        memalloc_nofs_restore(nofs);
}
EXPORT_SYMBOL_GPL(page_cache_readahead_unbounded);

/*
 * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates
 * the pages first, then submits them 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.
 */
void __do_page_cache_readahead(struct address_space *mapping,
                struct file *file, pgoff_t index, unsigned long nr_to_read,
                unsigned long lookahead_size)
{
        struct inode *inode = mapping->host;
        loff_t isize = i_size_read(inode);
        pgoff_t end_index;      /* The last page we want to read */

        if (isize == 0)
                return;

        end_index = (isize - 1) >> PAGE_SHIFT;
        if (index > end_index)
                return;
        /* Don't read past the page containing the last byte of the file */
        if (nr_to_read > end_index - index)
                nr_to_read = end_index - index + 1;

        page_cache_readahead_unbounded(mapping, file, index, nr_to_read,
                        lookahead_size);
}

/*
 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 * memory at once.
 */
void force_page_cache_readahead(struct address_space *mapping,
                struct file *filp, pgoff_t index, unsigned long nr_to_read)
{
        struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
        struct file_ra_state *ra = &filp->f_ra;
        unsigned long max_pages;

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

        /*
         * If the request exceeds the readahead window, allow the read to
         * be up to the optimal hardware IO size
         */
        max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
        nr_to_read = min(nr_to_read, max_pages);
        while (nr_to_read) {
                unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;

                if (this_chunk > nr_to_read)
                        this_chunk = nr_to_read;
                __do_page_cache_readahead(mapping, filp, index, this_chunk, 0);

                index += this_chunk;
                nr_to_read -= this_chunk;
        }
}

/*
 * 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;

        if (cur < max / 16)
                return 4 * cur;
        if (cur <= max / 2)
                return 2 * cur;
        return 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 @index-1 to @index-@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,
                                   pgoff_t index, unsigned long max)
{
        pgoff_t head;

        rcu_read_lock();
        head = page_cache_prev_miss(mapping, index - 1, max);
        rcu_read_unlock();

        return index - 1 - head;
}

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

        size = count_history_pages(mapping, index, max);

        /*
         * not enough history pages:
         * it could be a random read
         */
        if (size <= req_size)
                return 0;

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

        ra->start = index;
        ra->size = min(size + req_size, max);
        ra->async_size = 1;

        return 1;
}

/*
 * A minimal readahead algorithm for trivial sequential/random reads.
 */
static void ondemand_readahead(struct address_space *mapping,
                struct file_ra_state *ra, struct file *filp,
                bool hit_readahead_marker, pgoff_t index,
                unsigned long req_size)
{
        struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
        unsigned long max_pages = ra->ra_pages;
        unsigned long add_pages;
        pgoff_t prev_index;

        /*
         * If the request exceeds the readahead window, allow the read to
         * be up to the optimal hardware IO size
         */
        if (req_size > max_pages && bdi->io_pages > max_pages)
                max_pages = min(req_size, bdi->io_pages);

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

        /*
         * It's the expected callback index, assume sequential access.
         * Ramp up sizes, and push forward the readahead window.
         */
        if ((index == (ra->start + ra->size - ra->async_size) ||
             index == (ra->start + ra->size))) {
                ra->start += ra->size;
                ra->size = get_next_ra_size(ra, max_pages);
                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 = page_cache_next_miss(mapping, index + 1, max_pages);
                rcu_read_unlock();

                if (!start || start - index > max_pages)
                        return;

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

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

        /*
         * sequential cache miss
         * trivial case: (index - prev_index) == 1
         * unaligned reads: (index - prev_index) == 0
         */
        prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
        if (index - prev_index <= 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, index, req_size, max_pages))
                goto readit;

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

initial_readahead:
        ra->start = index;
        ra->size = get_init_ra_size(req_size, max_pages);
        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.
         * Take care of maximum IO pages as above.
         */
        if (index == ra->start && ra->size == ra->async_size) {
                add_pages = get_next_ra_size(ra, max_pages);
                if (ra->size + add_pages <= max_pages) {
                        ra->async_size = add_pages;
                        ra->size += add_pages;
                } else {
                        ra->size = max_pages;
                        ra->async_size = max_pages >> 1;
                }
        }

        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()
 * @index: Index of first page to be read.
 * @req_count: Total number of pages being read by the caller.
 *
 * 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 index, unsigned long req_count)
{
        /* no read-ahead */
        if (!ra->ra_pages)
                return;

        if (blk_cgroup_congested())
                return;

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

        /* do read-ahead */
        ondemand_readahead(mapping, ra, filp, false, index, req_count);
}
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 @index which triggered the readahead call.
 * @index: Index of first page to be read.
 * @req_count: Total number of pages being read by the caller.
 *
 * page_cache_async_readahead() should be called when a page is used which
 * is marked as PageReadahead; 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 index,
                           unsigned long req_count)
{
        /* 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 (inode_read_congested(mapping->host))
                return;

        if (blk_cgroup_congested())
                return;

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

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

        ret = -EBADF;
        f = fdget(fd);
        if (!f.file || !(f.file->f_mode & FMODE_READ))
                goto out;

        /*
         * The readahead() syscall is intended to run only on files
         * that can execute readahead. If readahead is not possible
         * on this file, then we must return -EINVAL.
         */
        ret = -EINVAL;
        if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
            !S_ISREG(file_inode(f.file)->i_mode))
                goto out;

        ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
out:
        fdput(f);
        return ret;
}

SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
{
        return ksys_readahead(fd, offset, count);
}