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SubjectRe: [PATCH] f2fs: fix to avoid potential deadlock
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On 2022/1/28 5:59, Jaegeuk Kim wrote:
> On 01/27, Chao Yu wrote:
>> Quoted from Jing Xia's report, there is a potential deadlock may happen
>> between kworker and checkpoint as below:
>>
>> [T:writeback] [T:checkpoint]
>> - wb_writeback
>> - blk_start_plug
>> bio contains NodeA was plugged in writeback threads
>
> I'm still trying to understand more precisely. So, how is it possible to
> have bio having node write in this current context?

IMO, after above blk_start_plug(), it may plug some inode's node page in kworker
during writebacking node_inode's data page (which should be node page)?

Thanks,

>
>> - do_writepages -- sync write inodeB, inc wb_sync_req[DATA]
>> - f2fs_write_data_pages
>> - f2fs_write_single_data_page -- write last dirty page
>> - f2fs_do_write_data_page
>> - set_page_writeback -- clear page dirty flag and
>> PAGECACHE_TAG_DIRTY tag in radix tree
>> - f2fs_outplace_write_data
>> - f2fs_update_data_blkaddr
>> - f2fs_wait_on_page_writeback -- wait NodeA to writeback here
>> - inode_dec_dirty_pages
>> - writeback_sb_inodes
>> - writeback_single_inode
>> - do_writepages
>> - f2fs_write_data_pages -- skip writepages due to wb_sync_req[DATA]
>> - wbc->pages_skipped += get_dirty_pages() -- PAGECACHE_TAG_DIRTY is not set but get_dirty_pages() returns one
>> - requeue_inode -- requeue inode to wb->b_dirty queue due to non-zero.pages_skipped
>> - blk_finish_plug
>>
>> Let's try to avoid deadlock condition by forcing unplugging previous bio via
>> blk_finish_plug(current->plug) once we'v skipped writeback in writepages()
>> due to valid sbi->wb_sync_req[DATA/NODE].
>>
>> Fixes: 687de7f1010c ("f2fs: avoid IO split due to mixed WB_SYNC_ALL and WB_SYNC_NONE")
>> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
>> Signed-off-by: Jing Xia <jing.xia@unisoc.com>
>> Signed-off-by: Chao Yu <chao@kernel.org>
>> ---
>> fs/f2fs/data.c | 6 +++++-
>> fs/f2fs/node.c | 6 +++++-
>> 2 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index 76d6fe7b0c8f..932a4c81acaf 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -3174,8 +3174,12 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>> /* to avoid spliting IOs due to mixed WB_SYNC_ALL and WB_SYNC_NONE */
>> if (wbc->sync_mode == WB_SYNC_ALL)
>> atomic_inc(&sbi->wb_sync_req[DATA]);
>> - else if (atomic_read(&sbi->wb_sync_req[DATA]))
>> + else if (atomic_read(&sbi->wb_sync_req[DATA])) {
>> + /* to avoid potential deadlock */
>> + if (current->plug)
>> + blk_finish_plug(current->plug);
>> goto skip_write;
>> + }
>>
>> if (__should_serialize_io(inode, wbc)) {
>> mutex_lock(&sbi->writepages);
>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>> index 556fcd8457f3..69c6bcaf5aae 100644
>> --- a/fs/f2fs/node.c
>> +++ b/fs/f2fs/node.c
>> @@ -2106,8 +2106,12 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>>
>> if (wbc->sync_mode == WB_SYNC_ALL)
>> atomic_inc(&sbi->wb_sync_req[NODE]);
>> - else if (atomic_read(&sbi->wb_sync_req[NODE]))
>> + else if (atomic_read(&sbi->wb_sync_req[NODE])) {
>> + /* to avoid potential deadlock */
>> + if (current->plug)
>> + blk_finish_plug(current->plug);
>> goto skip_write;
>> + }
>>
>> trace_f2fs_writepages(mapping->host, wbc, NODE);
>>
>> --
>> 2.32.0

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