/*
 * mm/page-writeback.c.
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains functions related to writing back dirty pages at the
 * address_space level.
 *
 * 10Apr2002    akpm@zip.com.au
 *              Initial version
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/spinlock.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/pagemap.h>
#include <linux/writeback.h>
#include <linux/init.h>
#include <linux/sysrq.h>
#include <linux/backing-dev.h>
#include <linux/mpage.h>
#include <linux/notifier.h>
#include <linux/smp.h>

/*
 * The maximum number of pages to writeout in a single bdflush/kupdate
 * operation.  We do this so we don't hold I_LOCK against an inode for
 * enormous amounts of time, which would block a userspace task which has
 * been forced to throttle against that inode.  Also, the code reevaluates
 * the dirty each time it has written this many pages.
 */
#define MAX_WRITEBACK_PAGES     1024

/*
 * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
 * will look to see if it needs to force writeback or throttling.
 */
static long ratelimit_pages = 32;

/*
 * The total number of pages in the machine.
 */
static long total_pages;

/*
 * When balance_dirty_pages decides that the caller needs to perform some
 * non-background writeback, this is how many pages it will attempt to write.
 * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably
 * large amounts of I/O are submitted.
 */
static inline long sync_writeback_pages(void)
{
        return ratelimit_pages + ratelimit_pages / 2;
}

/* The following parameters are exported via /proc/sys/vm */

/*
 * Dirty memory thresholds, in percentages
 */

/*
 * Start background writeback (via pdflush) at this level
 */
int dirty_background_ratio = 10;

/*
 * The generator of dirty data starts async writeback at this level
 */
int dirty_async_ratio = 40;

/*
 * The interval between `kupdate'-style writebacks, in centiseconds
 * (hundredths of a second)
 */
int dirty_writeback_centisecs = 5 * 100;

/*
 * The longest amount of time for which data is allowed to remain dirty
 */
int dirty_expire_centisecs = 30 * 100;

/* End of sysctl-exported parameters */


static void background_writeout(unsigned long _min_pages);

/*
 * balance_dirty_pages() must be called by processes which are
 * generating dirty data.  It looks at the number of dirty pages
 * in the machine and either:
 *
 * - Starts background writeback or
 * - Causes the caller to perform async writeback or
 * - Causes the caller to perform synchronous writeback, then
 *   tells a pdflush thread to perform more writeback or
 * - Does nothing at all.
 *
 * balance_dirty_pages() can sleep.
 */
void balance_dirty_pages(struct address_space *mapping)
{
        struct page_state ps;
        long background_thresh, async_thresh;
        unsigned long dirty_and_writeback;
        struct backing_dev_info *bdi;

        get_page_state(&ps);
        dirty_and_writeback = ps.nr_dirty + ps.nr_writeback;

        background_thresh = (dirty_background_ratio * total_pages) / 100;
        async_thresh = (dirty_async_ratio * total_pages) / 100;
        bdi = mapping->backing_dev_info;

        if (dirty_and_writeback > async_thresh) {
                struct writeback_control wbc = {
                        .bdi            = bdi,
                        .sync_mode      = WB_SYNC_NONE,
                        .older_than_this = NULL,
                        .nr_to_write    = sync_writeback_pages(),
                };

                writeback_inodes(&wbc);
                get_page_state(&ps);
        }

        if (!writeback_in_progress(bdi) && ps.nr_dirty > background_thresh)
                pdflush_operation(background_writeout, 0);
}
EXPORT_SYMBOL_GPL(balance_dirty_pages);

/**
 * balance_dirty_pages_ratelimited - balance dirty memory state
 * @mapping - address_space which was dirtied
 *
 * Processes which are dirtying memory should call in here once for each page
 * which was newly dirtied.  The function will periodically check the system's
 * dirty state and will initiate writeback if needed.
 *
 * balance_dirty_pages_ratelimited() may sleep.
 */
void balance_dirty_pages_ratelimited(struct address_space *mapping)
{
        static struct rate_limit_struct {
                int count;
        } ____cacheline_aligned ratelimits[NR_CPUS];
        int cpu;

        cpu = get_cpu();
        if (ratelimits[cpu].count++ >= ratelimit_pages) {
                ratelimits[cpu].count = 0;
                put_cpu();
                balance_dirty_pages(mapping);
                return;
        }
        put_cpu();
}

/*
 * writeback at least _min_pages, and keep writing until the amount of dirty
 * memory is less than the background threshold, or until we're all clean.
 */
static void background_writeout(unsigned long _min_pages)
{
        long min_pages = _min_pages;
        long background_thresh;
        struct writeback_control wbc = {
                .bdi            = NULL,
                .sync_mode      = WB_SYNC_NONE,
                .older_than_this = NULL,
                .nr_to_write    = 0,
        };

        CHECK_EMERGENCY_SYNC

        background_thresh = (dirty_background_ratio * total_pages) / 100;

        do {
                struct page_state ps;
                get_page_state(&ps);
                if (ps.nr_dirty < background_thresh && min_pages <= 0)
                        break;
                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
                writeback_inodes(&wbc);
                min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
        } while (wbc.nr_to_write <= 0);
        blk_run_queues();
}

/*
 * Start heavy writeback of everything.
 */
void wakeup_bdflush(void)
{
        struct page_state ps;

        get_page_state(&ps);
        pdflush_operation(background_writeout, ps.nr_dirty);
}

static struct timer_list wb_timer;

/*
 * Periodic writeback of "old" data.
 *
 * Define "old": the first time one of an inode's pages is dirtied, we mark the
 * dirtying-time in the inode's address_space.  So this periodic writeback code
 * just walks the superblock inode list, writing back any inodes which are
 * older than a specific point in time.
 *
 * Try to run once per dirty_writeback_centisecs.  But if a writeback event
 * takes longer than a dirty_writeback_centisecs interval, then leave a
 * one-second gap.
 *
 * older_than_this takes precedence over nr_to_write.  So we'll only write back
 * all dirty pages if they are all attached to "old" mappings.
 */
static void wb_kupdate(unsigned long arg)
{
        unsigned long oldest_jif;
        unsigned long start_jif;
        unsigned long next_jif;
        struct page_state ps;
        struct writeback_control wbc = {
                .bdi            = NULL,
                .sync_mode      = WB_SYNC_NONE,
                .older_than_this = &oldest_jif,
                .nr_to_write    = 0,
        };

        sync_supers();
        get_page_state(&ps);

        oldest_jif = jiffies - (dirty_expire_centisecs * HZ) / 100;
        start_jif = jiffies;
        next_jif = start_jif + (dirty_writeback_centisecs * HZ) / 100;
        wbc.nr_to_write = ps.nr_dirty;
        writeback_inodes(&wbc);
        blk_run_queues();
        yield();

        if (time_before(next_jif, jiffies + HZ))
                next_jif = jiffies + HZ;
        mod_timer(&wb_timer, next_jif);
}

static void wb_timer_fn(unsigned long unused)
{
        if (pdflush_operation(wb_kupdate, 0) < 0)
                mod_timer(&wb_timer, jiffies + HZ); /* delay 1 second */

}

/*
 * If ratelimit_pages is too high then we can get into dirty-data overload
 * if a large number of processes all perform writes at the same time.
 * If it is too low then SMP machines will call the (expensive) get_page_state
 * too often.
 *
 * Here we set ratelimit_pages to a level which ensures that when all CPUs are
 * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
 * thresholds before writeback cuts in.
 *
 * But the limit should not be set too high.  Because it also controls the
 * amount of memory which the balance_dirty_pages() caller has to write back.
 * If this is too large then the caller will block on the IO queue all the
 * time.  So limit it to four megabytes - the balance_dirty_pages() caller
 * will write six megabyte chunks, max.
 */

static void set_ratelimit(void)
{
        ratelimit_pages = total_pages / (num_online_cpus() * 32);
        if (ratelimit_pages < 16)
                ratelimit_pages = 16;
        if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
                ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
}

static int
ratelimit_handler(struct notifier_block *self, unsigned long u, void *v)
{
        set_ratelimit();
        return 0;
}

static struct notifier_block ratelimit_nb = {
        .notifier_call  = ratelimit_handler,
        .next           = NULL,
};

/*
 * If the machine has a large highmem:lowmem ratio then scale back the default
 * dirty memory thresholds: allowing too much dirty highmem pins an excessive
 * number of buffer_heads.
 */
static int __init page_writeback_init(void)
{
        long buffer_pages = nr_free_buffer_pages();
        long correction;

        total_pages = nr_free_pagecache_pages();

        correction = (100 * 4 * buffer_pages) / total_pages;

        if (correction < 100) {
                dirty_background_ratio *= correction;
                dirty_background_ratio /= 100;
                dirty_async_ratio *= correction;
                dirty_async_ratio /= 100;
        }

        init_timer(&wb_timer);
        wb_timer.expires = jiffies + (dirty_writeback_centisecs * HZ) / 100;
        wb_timer.data = 0;
        wb_timer.function = wb_timer_fn;
        add_timer(&wb_timer);
        set_ratelimit();
        register_cpu_notifier(&ratelimit_nb);
        return 0;
}
module_init(page_writeback_init);

/*
 * A library function, which implements the vm_writeback a_op.  It's fairly
 * lame at this time.  The idea is: the VM wants to liberate this page,
 * so we pass the page to the address_space and give the fs the opportunity
 * to write out lots of pages around this one.  It allows extent-based
 * filesytems to do intelligent things.  It lets delayed-allocate filesystems
 * perform better file layout.  It lets the address_space opportunistically
 * write back disk-contiguous pages which are in other zones.
 *
 * FIXME: the VM wants to start I/O against *this* page.  Because its zone
 * is under pressure.  But this function may start writeout against a
 * totally different set of pages.  Unlikely to be a huge problem, but if it
 * is, we could just writepage the page if it is still (PageDirty &&
 * !PageWriteback) (See below).
 *
 * Another option is to just reposition page->mapping->dirty_pages so we
 * *know* that the page will be written.  That will work fine, but seems
 * unpleasant.  (If the page is not for-sure on ->dirty_pages we're dead).
 * Plus it assumes that the address_space is performing writeback in
 * ->dirty_pages order.
 *
 * So.  The proper fix is to leave the page locked-and-dirty and to pass
 * it all the way down.
 */
int generic_vm_writeback(struct page *page, struct writeback_control *wbc)
{
        struct inode *inode = page->mapping->host;

        /*
         * We don't own this inode, and we don't want the address_space
         * vanishing while writeback is walking its pages.
         */
        inode = igrab(inode);
        unlock_page(page);

        if (inode) {
                do_writepages(inode->i_mapping, wbc);

                /*
                 * This iput() will internally call ext2_discard_prealloc(),
                 * which is rather bogus.  But there is no other way of
                 * dropping our ref to the inode.  However, there's no harm
                 * in dropping the prealloc, because there probably isn't any.
                 * Just a waste of cycles.
                 */
                iput(inode);
#if 0
                if (!PageWriteback(page) && PageDirty(page)) {
                        lock_page(page);
                        if (!PageWriteback(page)&&test_clear_page_dirty(page)) {
                                int ret;

                                ret = page->mapping->a_ops->writepage(page);
                                if (ret == -EAGAIN)
                                        __set_page_dirty_nobuffers(page);
                        } else {
                                unlock_page(page);
                        }
                }
#endif
        }
        return 0;
}
EXPORT_SYMBOL(generic_vm_writeback);

int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
{
        if (mapping->a_ops->writepages)
                return mapping->a_ops->writepages(mapping, wbc);
        return generic_writepages(mapping, wbc);
}

/**
 * write_one_page - write out a single page and optionally wait on I/O
 *
 * @page - the page to write
 * @wait - if true, wait on writeout
 *
 * The page must be locked by the caller and will be unlocked upon return.
 *
 * write_one_page() returns a negative error code if I/O failed.
 */
int write_one_page(struct page *page, int wait)
{
        struct address_space *mapping = page->mapping;
        int ret = 0;

        BUG_ON(!PageLocked(page));

        if (wait && PageWriteback(page))
                wait_on_page_writeback(page);

        write_lock(&mapping->page_lock);
        list_del(&page->list);
        if (test_clear_page_dirty(page)) {
                list_add(&page->list, &mapping->locked_pages);
                page_cache_get(page);
                write_unlock(&mapping->page_lock);
                ret = mapping->a_ops->writepage(page);
                if (ret == -EAGAIN) {
                        __set_page_dirty_nobuffers(page);
                        ret = 0;
                }
                if (ret == 0 && wait) {
                        wait_on_page_writeback(page);
                        if (PageError(page))
                                ret = -EIO;
                }
                page_cache_release(page);
        } else {
                list_add(&page->list, &mapping->clean_pages);
                write_unlock(&mapping->page_lock);
                unlock_page(page);
        }
        return ret;
}
EXPORT_SYMBOL(write_one_page);

/*
 * Add a page to the dirty page list.
 *
 * It is a sad fact of life that this function is called from several places
 * deeply under spinlocking.  It may not sleep.
 *
 * If the page has buffers, the uptodate buffers are set dirty, to preserve
 * dirty-state coherency between the page and the buffers.  It the page does
 * not have buffers then when they are later attached they will all be set
 * dirty.
 *
 * The buffers are dirtied before the page is dirtied.  There's a small race
 * window in which a writepage caller may see the page cleanness but not the
 * buffer dirtiness.  That's fine.  If this code were to set the page dirty
 * before the buffers, a concurrent writepage caller could clear the page dirty
 * bit, see a bunch of clean buffers and we'd end up with dirty buffers/clean
 * page on the dirty page list.
 *
 * There is also a small window where the page is dirty, and not on dirty_pages.
 * Also a possibility that by the time the page is added to dirty_pages, it has
 * been set clean.  The page lists are somewhat approximate in this regard.
 * It's better to have clean pages accidentally attached to dirty_pages than to
 * leave dirty pages attached to clean_pages.
 *
 * We use private_lock to lock against try_to_free_buffers while using the
 * page's buffer list.  Also use this to protect against clean buffers being
 * added to the page after it was set dirty.
 *
 * FIXME: may need to call ->reservepage here as well.  That's rather up to the
 * address_space though.
 *
 * For now, we treat swapper_space specially.  It doesn't use the normal
 * block a_ops.
 *
 * FIXME: this should move over to fs/buffer.c - buffer_heads have no business in mm/
 */
#include <linux/buffer_head.h>
int __set_page_dirty_buffers(struct page *page)
{
        struct address_space * const mapping = page->mapping;
        int ret = 0;

        if (mapping == NULL) {
                SetPageDirty(page);
                goto out;
        }

        if (!PageUptodate(page))
                buffer_error();

        spin_lock(&mapping->private_lock);

        if (page_has_buffers(page)) {
                struct buffer_head *head = page_buffers(page);
                struct buffer_head *bh = head;

                do {
                        if (buffer_uptodate(bh))
                                set_buffer_dirty(bh);
                        else
                                buffer_error();
                        bh = bh->b_this_page;
                } while (bh != head);
        }

        if (!TestSetPageDirty(page)) {
                write_lock(&mapping->page_lock);
                if (page->mapping) {    /* Race with truncate? */
                        if (!mapping->backing_dev_info->memory_backed)
                                inc_page_state(nr_dirty);
                        list_del(&page->list);
                        list_add(&page->list, &mapping->dirty_pages);
                }
                write_unlock(&mapping->page_lock);
                __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
        }
        
        spin_unlock(&mapping->private_lock);
out:
        return ret;
}
EXPORT_SYMBOL(__set_page_dirty_buffers);

/*
 * For address_spaces which do not use buffers.  Just set the page's dirty bit
 * and move it to the dirty_pages list.  Also perform space reservation if
 * required.
 *
 * __set_page_dirty_nobuffers() may return -ENOSPC.  But if it does, the page
 * is still safe, as long as it actually manages to find some blocks at
 * writeback time.
 *
 * This is also used when a single buffer is being dirtied: we want to set the
 * page dirty in that case, but not all the buffers.  This is a "bottom-up"
 * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
 */
int __set_page_dirty_nobuffers(struct page *page)
{
        int ret = 0;

        if (!TestSetPageDirty(page)) {
                struct address_space *mapping = page->mapping;

                if (mapping) {
                        write_lock(&mapping->page_lock);
                        if (page->mapping) {    /* Race with truncate? */
                                if (!mapping->backing_dev_info->memory_backed)
                                        inc_page_state(nr_dirty);
                                list_del(&page->list);
                                list_add(&page->list, &mapping->dirty_pages);
                        }
                        write_unlock(&mapping->page_lock);
                        __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
                }
        }
        return ret;
}
EXPORT_SYMBOL(__set_page_dirty_nobuffers);

/*
 * Clear a page's dirty flag, while caring for dirty memory accounting. 
 * Returns true if the page was previously dirty.
 */
int test_clear_page_dirty(struct page *page)
{
        if (TestClearPageDirty(page)) {
                struct address_space *mapping = page->mapping;

                if (mapping && !mapping->backing_dev_info->memory_backed)
                        dec_page_state(nr_dirty);
                return 1;
        }
        return 0;
}