[PATCH 5.4 217/411] fs-writeback: writeback_sb_inodes：Recalculate wrote according skipped pages

Greg Kroah-Hartman
2022-06-13T11:31:04Z

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
@@ -1650,11 +1650,12 @@ 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) {
@@ -1730,7 +1731,9 @@ 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()) {
/*
@@ -1752,7 +1755,7 @@ 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);
@@ -1766,14 +1769,14 @@ 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,