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    Subject[PATCH 14/17] writeback: make writeback_control.nr_to_write straight
    Pass struct wb_writeback_work all the way down to writeback_sb_inodes(),
    and initialize the struct writeback_control there.

    struct writeback_control is basically designed to control writeback of a
    single file, but we keep abuse it for writing multiple files in
    writeback_sb_inodes() and its callers.

    It immediately clean things up, e.g. suddenly wbc.nr_to_write vs
    work->nr_pages starts to make sense, and instead of saving and restoring
    pages_skipped in writeback_sb_inodes it can always start with a clean
    zero value.

    It also makes a neat IO pattern change: large dirty files are now
    written in the full 4MB writeback chunk size, rather than whatever
    remained quota in wbc->nr_to_write.

    Proposed-by: Christoph Hellwig <hch@infradead.org>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    fs/btrfs/extent_io.c | 2
    fs/fs-writeback.c | 184 +++++++++++++++--------------
    include/linux/writeback.h | 3
    include/trace/events/writeback.h | 6
    mm/backing-dev.c | 1
    mm/page-writeback.c | 1
    6 files changed, 99 insertions(+), 98 deletions(-)

    change set:
    - move writeback_control from wb_writeback() down to writeback_sb_inodes()
    - change return value of writeback_sb_inodes()/__writeback_inodes_wb()
    to the number of pages and/or inodes written
    - move writeback_control.older_than_this to struct wb_writeback_work
    - remove writeback_control.inodes_cleaned
    - remove wbc_writeback_* trace events for now

    --- linux-next.orig/fs/fs-writeback.c 2011-05-05 23:52:19.000000000 +0800
    +++ linux-next/fs/fs-writeback.c 2011-05-05 23:53:26.000000000 +0800
    @@ -30,11 +30,21 @@
    #include "internal.h"

    /*
    + * The maximum number of pages to writeout in a single bdi flush/kupdate
    + * operation. We do this so we don't hold I_SYNC 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 1024L
    +
    +/*
    * Passed into wb_writeback(), essentially a subset of writeback_control
    */
    struct wb_writeback_work {
    long nr_pages;
    struct super_block *sb;
    + unsigned long *older_than_this;
    enum writeback_sync_modes sync_mode;
    unsigned int tagged_sync:1;
    unsigned int for_kupdate:1;
    @@ -463,7 +473,6 @@ writeback_single_inode(struct inode *ino
    * No need to add it back to the LRU.
    */
    list_del_init(&inode->i_wb_list);
    - wbc->inodes_cleaned++;
    }
    }
    inode_sync_complete(inode);
    @@ -496,6 +505,31 @@ static bool pin_sb_for_writeback(struct
    return false;
    }

    +static long writeback_chunk_size(struct wb_writeback_work *work)
    +{
    + long pages;
    +
    + /*
    + * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
    + * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
    + * here avoids calling into writeback_inodes_wb() more than once.
    + *
    + * The intended call sequence for WB_SYNC_ALL writeback is:
    + *
    + * wb_writeback()
    + * writeback_sb_inodes() <== called only once
    + * write_cache_pages() <== called once for each inode
    + * (quickly) tag currently dirty pages
    + * (maybe slowly) sync all tagged pages
    + */
    + if (work->sync_mode == WB_SYNC_ALL || work->tagged_sync)
    + pages = LONG_MAX;
    + else
    + pages = min(MAX_WRITEBACK_PAGES, work->nr_pages);
    +
    + return pages;
    +}
    +
    /*
    * Write a portion of b_io inodes which belong to @sb.
    *
    @@ -503,18 +537,29 @@ static bool pin_sb_for_writeback(struct
    * inodes. Otherwise write only ones which go sequentially
    * in reverse order.
    *
    - * Return 1, if the caller writeback routine should be
    - * interrupted. Otherwise return 0.
    + * Return the number of pages and/or inodes cleaned.
    */
    -static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
    - struct writeback_control *wbc, bool only_this_sb)
    +static long writeback_sb_inodes(struct super_block *sb,
    + struct bdi_writeback *wb,
    + struct wb_writeback_work *work)
    {
    + struct writeback_control wbc = {
    + .sync_mode = work->sync_mode,
    + .tagged_sync = work->tagged_sync,
    + .for_kupdate = work->for_kupdate,
    + .for_background = work->for_background,
    + .range_cyclic = work->range_cyclic,
    + .range_start = 0,
    + .range_end = LLONG_MAX,
    + };
    + long write_chunk;
    + long wrote = 0; /* count both pages and inodes */
    +
    while (!list_empty(&wb->b_io)) {
    - long pages_skipped;
    struct inode *inode = wb_inode(wb->b_io.prev);

    if (inode->i_sb != sb) {
    - if (only_this_sb) {
    + if (work->sb) {
    /*
    * We only want to write back data for this
    * superblock, move all inodes not belonging
    @@ -529,7 +574,7 @@ static int writeback_sb_inodes(struct su
    * Bounce back to the caller to unpin this and
    * pin the next superblock.
    */
    - return 0;
    + break;
    }

    /*
    @@ -543,34 +588,40 @@ static int writeback_sb_inodes(struct su
    requeue_io(inode, wb);
    continue;
    }
    -
    __iget(inode);
    + write_chunk = writeback_chunk_size(work);
    + wbc.nr_to_write = write_chunk;
    + wbc.pages_skipped = 0;
    +
    + writeback_single_inode(inode, wb, &wbc);

    - pages_skipped = wbc->pages_skipped;
    - writeback_single_inode(inode, wb, wbc);
    - if (wbc->pages_skipped != pages_skipped) {
    + work->nr_pages -= write_chunk - wbc.nr_to_write;
    + wrote += write_chunk - wbc.nr_to_write;
    + if (wbc.pages_skipped) {
    /*
    * writeback is not making progress due to locked
    * buffers. Skip this inode for now.
    */
    redirty_tail(inode, wb);
    - }
    + } else if (!(inode->i_state & I_DIRTY))
    + wrote++;
    spin_unlock(&inode->i_lock);
    spin_unlock(&wb->list_lock);
    iput(inode);
    cond_resched();
    spin_lock(&wb->list_lock);
    - if (wbc->nr_to_write <= 0)
    - return 1;
    + if (wrote >= MAX_WRITEBACK_PAGES)
    + break;
    + if (work->nr_pages <= 0)
    + break;
    }
    - /* b_io is empty */
    - return 1;
    + return wrote;
    }

    -static void __writeback_inodes_wb(struct bdi_writeback *wb,
    - struct writeback_control *wbc)
    +static long __writeback_inodes_wb(struct bdi_writeback *wb,
    + struct wb_writeback_work *work)
    {
    - int ret = 0;
    + long wrote = 0;

    while (!list_empty(&wb->b_io)) {
    struct inode *inode = wb_inode(wb->b_io.prev);
    @@ -580,33 +631,35 @@ static void __writeback_inodes_wb(struct
    requeue_io(inode, wb);
    continue;
    }
    - ret = writeback_sb_inodes(sb, wb, wbc, false);
    + wrote += writeback_sb_inodes(sb, wb, work);
    drop_super(sb);

    - if (ret)
    + if (wrote >= MAX_WRITEBACK_PAGES)
    + break;
    + if (work->nr_pages <= 0)
    break;
    }
    /* Leave any unwritten inodes on b_io */
    + return wrote;
    }

    void writeback_inodes_wb(struct bdi_writeback *wb,
    - struct writeback_control *wbc)
    + struct writeback_control *wbc)
    {
    + struct wb_writeback_work work = {
    + .nr_pages = wbc->nr_to_write,
    + .sync_mode = wbc->sync_mode,
    + .range_cyclic = wbc->range_cyclic,
    + };
    +
    spin_lock(&wb->list_lock);
    if (list_empty(&wb->b_io))
    - queue_io(wb, wbc->older_than_this);
    - __writeback_inodes_wb(wb, wbc);
    + queue_io(wb, NULL);
    + __writeback_inodes_wb(wb, &work);
    spin_unlock(&wb->list_lock);
    -}

    -/*
    - * The maximum number of pages to writeout in a single bdi flush/kupdate
    - * operation. We do this so we don't hold I_SYNC 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
    + wbc->nr_to_write = work.nr_pages;
    +}

    static inline bool over_bground_thresh(void)
    {
    @@ -636,42 +689,13 @@ static inline bool over_bground_thresh(v
    static long wb_writeback(struct bdi_writeback *wb,
    struct wb_writeback_work *work)
    {
    - struct writeback_control wbc = {
    - .sync_mode = work->sync_mode,
    - .tagged_sync = work->tagged_sync,
    - .older_than_this = NULL,
    - .for_kupdate = work->for_kupdate,
    - .for_background = work->for_background,
    - .range_cyclic = work->range_cyclic,
    - };
    + long nr_pages = work->nr_pages;
    unsigned long oldest_jif;
    - long wrote = 0;
    - long write_chunk = MAX_WRITEBACK_PAGES;
    struct inode *inode;
    -
    - if (!wbc.range_cyclic) {
    - wbc.range_start = 0;
    - wbc.range_end = LLONG_MAX;
    - }
    -
    - /*
    - * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
    - * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
    - * here avoids calling into writeback_inodes_wb() more than once.
    - *
    - * The intended call sequence for WB_SYNC_ALL writeback is:
    - *
    - * wb_writeback()
    - * writeback_sb_inodes() <== called only once
    - * write_cache_pages() <== called once for each inode
    - * (quickly) tag currently dirty pages
    - * (maybe slowly) sync all tagged pages
    - */
    - if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync)
    - write_chunk = LONG_MAX;
    + long progress;

    oldest_jif = jiffies;
    - wbc.older_than_this = &oldest_jif;
    + work->older_than_this = &oldest_jif;

    for (;;) {
    /*
    @@ -700,27 +724,18 @@ static long wb_writeback(struct bdi_writ
    if (work->for_kupdate || work->for_background) {
    oldest_jif = jiffies -
    msecs_to_jiffies(dirty_expire_interval * 10);
    - wbc.older_than_this = &oldest_jif;
    + work->older_than_this = &oldest_jif;
    }

    - wbc.nr_to_write = write_chunk;
    - wbc.pages_skipped = 0;
    - wbc.inodes_cleaned = 0;
    -
    retry:
    - trace_wbc_writeback_start(&wbc, wb->bdi);
    spin_lock(&wb->list_lock);
    if (list_empty(&wb->b_io))
    - queue_io(wb, wbc.older_than_this);
    + queue_io(wb, work->older_than_this);
    if (work->sb)
    - writeback_sb_inodes(work->sb, wb, &wbc, true);
    + progress = writeback_sb_inodes(work->sb, wb, work);
    else
    - __writeback_inodes_wb(wb, &wbc);
    + progress = __writeback_inodes_wb(wb, work);
    spin_unlock(&wb->list_lock);
    - trace_wbc_writeback_written(&wbc, wb->bdi);
    -
    - work->nr_pages -= write_chunk - wbc.nr_to_write;
    - wrote += write_chunk - wbc.nr_to_write;

    /*
    * Did we write something? Try for more
    @@ -730,9 +745,7 @@ retry:
    * mean the overall work is done. So we keep looping as long
    * as made some progress on cleaning pages or inodes.
    */
    - if (wbc.nr_to_write < write_chunk)
    - continue;
    - if (wbc.inodes_cleaned)
    + if (progress)
    continue;
    /*
    * background writeback will start with expired inodes, and
    @@ -741,10 +754,10 @@ retry:
    * lists and cause trouble to the page reclaim.
    */
    if (work->for_background &&
    - wbc.older_than_this &&
    + work->older_than_this &&
    list_empty(&wb->b_io) &&
    list_empty(&wb->b_more_io)) {
    - wbc.older_than_this = NULL;
    + work->older_than_this = NULL;
    goto retry;
    }
    /*
    @@ -760,7 +773,6 @@ retry:
    spin_lock(&wb->list_lock);
    if (!list_empty(&wb->b_more_io)) {
    inode = wb_inode(wb->b_more_io.prev);
    - trace_wbc_writeback_wait(&wbc, wb->bdi);
    spin_lock(&inode->i_lock);
    inode_wait_for_writeback(inode, wb);
    spin_unlock(&inode->i_lock);
    @@ -768,7 +780,7 @@ retry:
    spin_unlock(&wb->list_lock);
    }

    - return wrote;
    + return nr_pages - work->nr_pages;
    }

    /*
    --- linux-next.orig/include/linux/writeback.h 2011-05-05 23:52:19.000000000 +0800
    +++ linux-next/include/linux/writeback.h 2011-05-05 23:53:26.000000000 +0800
    @@ -24,12 +24,9 @@ enum writeback_sync_modes {
    */
    struct writeback_control {
    enum writeback_sync_modes sync_mode;
    - unsigned long *older_than_this; /* If !NULL, only write back inodes
    - older than this */
    long nr_to_write; /* Write this many pages, and decrement
    this for each page written */
    long pages_skipped; /* Pages which were not written */
    - long inodes_cleaned; /* # of inodes cleaned */

    /*
    * For a_ops->writepages(): is start or end are non-zero then this is
    --- linux-next.orig/mm/backing-dev.c 2011-05-05 23:41:35.000000000 +0800
    +++ linux-next/mm/backing-dev.c 2011-05-05 23:53:26.000000000 +0800
    @@ -264,7 +264,6 @@ static void bdi_flush_io(struct backing_
    {
    struct writeback_control wbc = {
    .sync_mode = WB_SYNC_NONE,
    - .older_than_this = NULL,
    .range_cyclic = 1,
    .nr_to_write = 1024,
    };
    --- linux-next.orig/mm/page-writeback.c 2011-05-05 23:52:18.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2011-05-05 23:53:26.000000000 +0800
    @@ -496,7 +496,6 @@ static void balance_dirty_pages(struct a
    for (;;) {
    struct writeback_control wbc = {
    .sync_mode = WB_SYNC_NONE,
    - .older_than_this = NULL,
    .nr_to_write = write_chunk,
    .range_cyclic = 1,
    };
    --- linux-next.orig/include/trace/events/writeback.h 2011-05-05 23:52:19.000000000 +0800
    +++ linux-next/include/trace/events/writeback.h 2011-05-05 23:53:26.000000000 +0800
    @@ -101,7 +101,6 @@ DECLARE_EVENT_CLASS(wbc_class,
    __field(int, for_background)
    __field(int, for_reclaim)
    __field(int, range_cyclic)
    - __field(unsigned long, older_than_this)
    __field(long, range_start)
    __field(long, range_end)
    ),
    @@ -115,14 +114,12 @@ DECLARE_EVENT_CLASS(wbc_class,
    __entry->for_background = wbc->for_background;
    __entry->for_reclaim = wbc->for_reclaim;
    __entry->range_cyclic = wbc->range_cyclic;
    - __entry->older_than_this = wbc->older_than_this ?
    - *wbc->older_than_this : 0;
    __entry->range_start = (long)wbc->range_start;
    __entry->range_end = (long)wbc->range_end;
    ),

    TP_printk("bdi %s: towrt=%ld skip=%ld mode=%d kupd=%d "
    - "bgrd=%d reclm=%d cyclic=%d older=0x%lx "
    + "bgrd=%d reclm=%d cyclic=%d "
    "start=0x%lx end=0x%lx",
    __entry->name,
    __entry->nr_to_write,
    @@ -132,7 +129,6 @@ DECLARE_EVENT_CLASS(wbc_class,
    __entry->for_background,
    __entry->for_reclaim,
    __entry->range_cyclic,
    - __entry->older_than_this,
    __entry->range_start,
    __entry->range_end)
    )
    --- linux-next.orig/fs/btrfs/extent_io.c 2011-05-05 23:41:35.000000000 +0800
    +++ linux-next/fs/btrfs/extent_io.c 2011-05-05 23:53:26.000000000 +0800
    @@ -2587,7 +2587,6 @@ int extent_write_full_page(struct extent
    };
    struct writeback_control wbc_writepages = {
    .sync_mode = wbc->sync_mode,
    - .older_than_this = NULL,
    .nr_to_write = 64,
    .range_start = page_offset(page) + PAGE_CACHE_SIZE,
    .range_end = (loff_t)-1,
    @@ -2620,7 +2619,6 @@ int extent_write_locked_range(struct ext
    };
    struct writeback_control wbc_writepages = {
    .sync_mode = mode,
    - .older_than_this = NULL,
    .nr_to_write = nr_pages * 2,
    .range_start = start,
    .range_end = end + 1,



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