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    Subject[PATCH 1/3] writeback: introduce wbc.for_sync to cover the two sync stages
    sync(2) is performed in two stages: the WB_SYNC_NONE sync and the
    WB_SYNC_ALL sync. Tag both stages with wbc.for_sync for livelock
    prevention.

    Note that writeback_inodes_sb() is called by not only sync(), they
    are treated the same because the other callers need also need livelock
    prevention.

    Impacts:

    - it changes the order in which pages/inodes are synced to disk. Now in
    the WB_SYNC_NONE stage, it won't proceed to write the next inode until
    finished with the current inode.

    - this adds a new field to the writeback trace events and may possibly
    break some scripts.

    CC: Jan Kara <jack@suse.cz>
    CC: Dave Chinner <david@fromorbit.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    fs/ext4/inode.c | 4 ++--
    fs/fs-writeback.c | 10 ++++++----
    include/linux/writeback.h | 1 +
    include/trace/events/writeback.h | 10 ++++++++--
    mm/page-writeback.c | 4 ++--
    5 files changed, 19 insertions(+), 10 deletions(-)

    --- linux-next.orig/fs/fs-writeback.c 2011-05-01 06:35:15.000000000 +0800
    +++ linux-next/fs/fs-writeback.c 2011-05-01 06:35:17.000000000 +0800
    @@ -36,6 +36,7 @@ struct wb_writeback_work {
    long nr_pages;
    struct super_block *sb;
    enum writeback_sync_modes sync_mode;
    + unsigned int for_sync:1;
    unsigned int for_kupdate:1;
    unsigned int range_cyclic:1;
    unsigned int for_background:1;
    @@ -645,13 +646,14 @@ static long wb_writeback(struct bdi_writ
    struct writeback_control wbc = {
    .sync_mode = work->sync_mode,
    .older_than_this = NULL,
    + .for_sync = work->for_sync,
    .for_kupdate = work->for_kupdate,
    .for_background = work->for_background,
    .range_cyclic = work->range_cyclic,
    };
    unsigned long oldest_jif;
    long wrote = 0;
    - long write_chunk;
    + long write_chunk = MAX_WRITEBACK_PAGES;
    struct inode *inode;

    if (!wbc.range_cyclic) {
    @@ -672,9 +674,7 @@ static long wb_writeback(struct bdi_writ
    * (quickly) tag currently dirty pages
    * (maybe slowly) sync all tagged pages
    */
    - if (wbc.sync_mode == WB_SYNC_NONE)
    - write_chunk = MAX_WRITEBACK_PAGES;
    - else
    + if (wbc.for_sync)
    write_chunk = LONG_MAX;

    wbc.wb_start = jiffies; /* livelock avoidance */
    @@ -1209,6 +1209,7 @@ void writeback_inodes_sb_nr(struct super
    struct wb_writeback_work work = {
    .sb = sb,
    .sync_mode = WB_SYNC_NONE,
    + .for_sync = 1,
    .done = &done,
    .nr_pages = nr,
    };
    @@ -1286,6 +1287,7 @@ void sync_inodes_sb(struct super_block *
    struct wb_writeback_work work = {
    .sb = sb,
    .sync_mode = WB_SYNC_ALL,
    + .for_sync = 1,
    .nr_pages = LONG_MAX,
    .range_cyclic = 0,
    .done = &done,
    --- linux-next.orig/include/linux/writeback.h 2011-05-01 06:35:16.000000000 +0800
    +++ linux-next/include/linux/writeback.h 2011-05-01 06:35:17.000000000 +0800
    @@ -46,6 +46,7 @@ struct writeback_control {
    unsigned encountered_congestion:1; /* An output: a queue is full */
    unsigned for_kupdate:1; /* A kupdate writeback */
    unsigned for_background:1; /* A background writeback */
    + unsigned for_sync:1; /* do livelock prevention for sync */
    unsigned for_reclaim:1; /* Invoked from the page allocator */
    unsigned range_cyclic:1; /* range_start is cyclic */
    unsigned more_io:1; /* more io to be dispatched */
    --- linux-next.orig/mm/page-writeback.c 2011-05-01 06:35:16.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2011-05-01 06:35:17.000000000 +0800
    @@ -892,12 +892,12 @@ int write_cache_pages(struct address_spa
    range_whole = 1;
    cycled = 1; /* ignore range_cyclic tests */
    }
    - if (wbc->sync_mode == WB_SYNC_ALL)
    + if (wbc->for_sync)
    tag = PAGECACHE_TAG_TOWRITE;
    else
    tag = PAGECACHE_TAG_DIRTY;
    retry:
    - if (wbc->sync_mode == WB_SYNC_ALL)
    + if (wbc->for_sync)
    tag_pages_for_writeback(mapping, index, end);
    done_index = index;
    while (!done && (index <= end)) {
    --- linux-next.orig/include/trace/events/writeback.h 2011-05-01 06:35:16.000000000 +0800
    +++ linux-next/include/trace/events/writeback.h 2011-05-01 06:35:17.000000000 +0800
    @@ -17,6 +17,7 @@ DECLARE_EVENT_CLASS(writeback_work_class
    __array(char, name, 32)
    __field(long, nr_pages)
    __field(dev_t, sb_dev)
    + __field(int, for_sync)
    __field(int, sync_mode)
    __field(int, for_kupdate)
    __field(int, range_cyclic)
    @@ -26,16 +27,18 @@ DECLARE_EVENT_CLASS(writeback_work_class
    strncpy(__entry->name, dev_name(bdi->dev), 32);
    __entry->nr_pages = work->nr_pages;
    __entry->sb_dev = work->sb ? work->sb->s_dev : 0;
    + __entry->for_sync = work->for_sync;
    __entry->sync_mode = work->sync_mode;
    __entry->for_kupdate = work->for_kupdate;
    __entry->range_cyclic = work->range_cyclic;
    __entry->for_background = work->for_background;
    ),
    - TP_printk("bdi %s: sb_dev %d:%d nr_pages=%ld sync_mode=%d "
    + TP_printk("bdi %s: sb_dev %d:%d nr_pages=%ld sync=%d sync_mode=%d "
    "kupdate=%d range_cyclic=%d background=%d",
    __entry->name,
    MAJOR(__entry->sb_dev), MINOR(__entry->sb_dev),
    __entry->nr_pages,
    + __entry->for_sync,
    __entry->sync_mode,
    __entry->for_kupdate,
    __entry->range_cyclic,
    @@ -96,6 +99,7 @@ DECLARE_EVENT_CLASS(wbc_class,
    __array(char, name, 32)
    __field(long, nr_to_write)
    __field(long, pages_skipped)
    + __field(int, for_sync)
    __field(int, sync_mode)
    __field(int, for_kupdate)
    __field(int, for_background)
    @@ -111,6 +115,7 @@ DECLARE_EVENT_CLASS(wbc_class,
    strncpy(__entry->name, dev_name(bdi->dev), 32);
    __entry->nr_to_write = wbc->nr_to_write;
    __entry->pages_skipped = wbc->pages_skipped;
    + __entry->for_sync = wbc->for_sync;
    __entry->sync_mode = wbc->sync_mode;
    __entry->for_kupdate = wbc->for_kupdate;
    __entry->for_background = wbc->for_background;
    @@ -123,12 +128,13 @@ DECLARE_EVENT_CLASS(wbc_class,
    __entry->range_end = (long)wbc->range_end;
    ),

    - TP_printk("bdi %s: towrt=%ld skip=%ld mode=%d kupd=%d "
    + TP_printk("bdi %s: towrt=%ld skip=%ld sync=%d mode=%d kupd=%d "
    "bgrd=%d reclm=%d cyclic=%d more=%d older=0x%lx "
    "start=0x%lx end=0x%lx",
    __entry->name,
    __entry->nr_to_write,
    __entry->pages_skipped,
    + __entry->for_sync,
    __entry->sync_mode,
    __entry->for_kupdate,
    __entry->for_background,
    --- linux-next.orig/fs/ext4/inode.c 2011-05-01 06:35:15.000000000 +0800
    +++ linux-next/fs/ext4/inode.c 2011-05-01 06:35:17.000000000 +0800
    @@ -2741,7 +2741,7 @@ static int write_cache_pages_da(struct a
    index = wbc->range_start >> PAGE_CACHE_SHIFT;
    end = wbc->range_end >> PAGE_CACHE_SHIFT;

    - if (wbc->sync_mode == WB_SYNC_ALL)
    + if (wbc->for_sync)
    tag = PAGECACHE_TAG_TOWRITE;
    else
    tag = PAGECACHE_TAG_DIRTY;
    @@ -2975,7 +2975,7 @@ static int ext4_da_writepages(struct add
    }

    retry:
    - if (wbc->sync_mode == WB_SYNC_ALL)
    + if (wbc->for_sync)
    tag_pages_for_writeback(mapping, index, end);

    while (!ret && wbc->nr_to_write > 0) {



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