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    Subject[PATCH 4.9 077/167] fs-writeback: writeback sb inodes:Recalculate wrote according skipped pages
    Date
    From: Zhihao Cheng <chengzhihao1@huawei.com>

    commit 68f4c6eba70df70a720188bce95c85570ddfcc87 upstream.

    Commit 505a666ee3fc ("writeback: plug writeback in wb_writeback() and
    writeback_inodes_wb()") has us holding a plug during wb_writeback, which
    may cause a potential ABBA dead lock:

    wb_writeback fat_file_fsync
    blk_start_plug(&plug)
    for (;;) {
    iter i-1: some reqs have been added into plug->mq_list // LOCK A
    iter i:
    progress = __writeback_inodes_wb(wb, work)
    . writeback_sb_inodes // fat's bdev
    . __writeback_single_inode
    . . generic_writepages
    . . __block_write_full_page
    . . . . __generic_file_fsync
    . . . . sync_inode_metadata
    . . . . writeback_single_inode
    . . . . __writeback_single_inode
    . . . . fat_write_inode
    . . . . __fat_write_inode
    . . . . sync_dirty_buffer // fat's bdev
    . . . . lock_buffer(bh) // LOCK B
    . . . . submit_bh
    . . . . blk_mq_get_tag // LOCK A
    . . . trylock_buffer(bh) // LOCK B
    . . . redirty_page_for_writepage
    . . . wbc->pages_skipped++
    . . --wbc->nr_to_write
    . wrote += write_chunk - wbc.nr_to_write // wrote > 0
    . requeue_inode
    . redirty_tail_locked
    if (progress) // progress > 0
    continue;
    iter i+1:
    queue_io
    // similar process with iter i, infinite for-loop !
    }
    blk_finish_plug(&plug) // flush plug won't be called

    Above process triggers a hungtask like:
    [ 399.044861] INFO: task bb:2607 blocked for more than 30 seconds.
    [ 399.046824] Not tainted 5.18.0-rc1-00005-gefae4d9eb6a2-dirty
    [ 399.051539] task:bb state:D stack: 0 pid: 2607 ppid:
    2426 flags:0x00004000
    [ 399.051556] Call Trace:
    [ 399.051570] __schedule+0x480/0x1050
    [ 399.051592] schedule+0x92/0x1a0
    [ 399.051602] io_schedule+0x22/0x50
    [ 399.051613] blk_mq_get_tag+0x1d3/0x3c0
    [ 399.051640] __blk_mq_alloc_requests+0x21d/0x3f0
    [ 399.051657] blk_mq_submit_bio+0x68d/0xca0
    [ 399.051674] __submit_bio+0x1b5/0x2d0
    [ 399.051708] submit_bio_noacct+0x34e/0x720
    [ 399.051718] submit_bio+0x3b/0x150
    [ 399.051725] submit_bh_wbc+0x161/0x230
    [ 399.051734] __sync_dirty_buffer+0xd1/0x420
    [ 399.051744] sync_dirty_buffer+0x17/0x20
    [ 399.051750] __fat_write_inode+0x289/0x310
    [ 399.051766] fat_write_inode+0x2a/0xa0
    [ 399.051783] __writeback_single_inode+0x53c/0x6f0
    [ 399.051795] writeback_single_inode+0x145/0x200
    [ 399.051803] sync_inode_metadata+0x45/0x70
    [ 399.051856] __generic_file_fsync+0xa3/0x150
    [ 399.051880] fat_file_fsync+0x1d/0x80
    [ 399.051895] vfs_fsync_range+0x40/0xb0
    [ 399.051929] __x64_sys_fsync+0x18/0x30

    In my test, 'need_resched()' (which is imported by 590dca3a71 "fs-writeback:
    unplug before cond_resched in writeback_sb_inodes") in function
    'writeback_sb_inodes()' seldom comes true, unless cond_resched() is deleted
    from write_cache_pages().

    Fix it by correcting wrote number according number of skipped pages
    in writeback_sb_inodes().

    Goto Link to find a reproducer.

    Link: https://bugzilla.kernel.org/show_bug.cgi?id=215837
    Cc: stable@vger.kernel.org # v4.3
    Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Link: https://lore.kernel.org/r/20220510133805.1988292-1-chengzhihao1@huawei.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/fs-writeback.c | 13 ++++++++-----
    1 file changed, 8 insertions(+), 5 deletions(-)

    --- a/fs/fs-writeback.c
    +++ b/fs/fs-writeback.c
    @@ -1569,11 +1569,12 @@ static long writeback_sb_inodes(struct s
    };
    unsigned long start_time = jiffies;
    long write_chunk;
    - long wrote = 0; /* count both pages and inodes */
    + long total_wrote = 0; /* count both pages and inodes */

    while (!list_empty(&wb->b_io)) {
    struct inode *inode = wb_inode(wb->b_io.prev);
    struct bdi_writeback *tmp_wb;
    + long wrote;

    if (inode->i_sb != sb) {
    if (work->sb) {
    @@ -1649,7 +1650,9 @@ static long writeback_sb_inodes(struct s

    wbc_detach_inode(&wbc);
    work->nr_pages -= write_chunk - wbc.nr_to_write;
    - wrote += write_chunk - wbc.nr_to_write;
    + wrote = write_chunk - wbc.nr_to_write - wbc.pages_skipped;
    + wrote = wrote < 0 ? 0 : wrote;
    + total_wrote += wrote;

    if (need_resched()) {
    /*
    @@ -1671,7 +1674,7 @@ static long writeback_sb_inodes(struct s
    tmp_wb = inode_to_wb_and_lock_list(inode);
    spin_lock(&inode->i_lock);
    if (!(inode->i_state & I_DIRTY_ALL))
    - wrote++;
    + total_wrote++;
    requeue_inode(inode, tmp_wb, &wbc);
    inode_sync_complete(inode);
    spin_unlock(&inode->i_lock);
    @@ -1685,14 +1688,14 @@ static long writeback_sb_inodes(struct s
    * bail out to wb_writeback() often enough to check
    * background threshold and other termination conditions.
    */
    - if (wrote) {
    + if (total_wrote) {
    if (time_is_before_jiffies(start_time + HZ / 10UL))
    break;
    if (work->nr_pages <= 0)
    break;
    }
    }
    - return wrote;
    + return total_wrote;
    }

    static long __writeback_inodes_wb(struct bdi_writeback *wb,

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