Messages in this thread Patch in this message | | | Date | Sat, 18 Dec 2021 15:38:16 +0800 | Subject | Re: [f2fs-dev] [PATCH Vx 1/1] f2fs: Avoid deadlock between writeback and checkpoint | From | Chao Yu <> |
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On 2021/12/18 0:59, Jaegeuk Kim wrote: > On 12/17, Chao Yu wrote: >> On 2021/11/20 9:50, Chao Yu wrote: >>> On 2021/11/18 14:46, Chao Yu wrote: >>>> On 2021/11/18 0:56, Jaegeuk Kim wrote: >>>>> On 11/09, niuzhiguo84@gmail.com wrote: >>>>>> From: Zhiguo Niu <zhiguo.niu@unisoc.com> >>>>>> >>>>>> There could be a scenario as following: >>>>>> The inodeA and inodeB are in b_io queue of writeback >>>>>> inodeA : f2fs's node inode >>>>>> inodeB : a dir inode with only one dirty pages, and the node page >>>>>> of inodeB cached into inodeA >>>>>> >>>>>> writeback: >>>>>> >>>>>> wb_workfn >>>>>> wb_writeback >>>>>> blk_start_plug >>>>>> loop { >>>>>> queue_io >>>>>> progress=__writeback_inodes_wb >>>>>> __writeback_single_inode >>>>>> do_writepages >>>>>> f2fs_write_data_pages >>>>>> wbc->pages_skipped +=get_dirty_pages >>>>>> inode->i_state &= ~dirty >>>>>> wrote++ >>>>>> requeue_inode >>>>>> } >>>>>> blk_finish_plug >>>>>> >>>>>> checkpoint: >>>>>> >>>>>> f2fs_write_checkpoint >>>>>> f2fs_sync_dirty_inodes >>>>>> filemap_fdatawrite >>>>>> do_writepages >>>>>> f2fs_write_data_pages >>>>>> f2fs_write_single_data_page >>>>>> f2fs_do_write_data_page >>>>>> set_page_writeback >>>>>> f2fs_outplace_write_data >>>>>> f2fs_update_data_blkaddr >>>>>> f2fs_wait_on_page_writeback >>>>>> inode_dec_dirty_pages >>>>>> >>>>>> 1. Writeback thread flush inodeA, and push it's bio request in task's plug; >>>>>> 2. Checkpoint thread writes inodeB's dirty page, and then wait its node >>>>>> page writeback cached into inodeA which is in writeback task's plug >>>>>> 3. Writeback thread flush inodeB and skip writing the dirty page as >>>>>> wb_sync_req[DATA] > 0. >>>>>> 4. As none of the inodeB's page is marked as PAGECACHE_TAG_DIRTY, writeback >>>>>> thread clear inodeB's dirty state. >>>>>> 5. Then inodeB is moved from b_io to b_dirty because of pages_skipped > 0 >>>>>> as checkpoint thread is stuck before dec dirty_pages. >>>>>> >>>>>> This patch collect correct pages_skipped according to the tag state in >>>>>> page tree of inode >>>>>> >>>>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com> >>>>>> Signed-off-by: Jing Xia <jing.xia@unisoc.com> >>>>>> --- >>>>>> fs/f2fs/data.c | 4 +++- >>>>>> 1 file changed, 3 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c >>>>>> index f4fd6c246c9a..e98628e3868c 100644 >>>>>> --- a/fs/f2fs/data.c >>>>>> +++ b/fs/f2fs/data.c >>>>>> @@ -3237,7 +3237,9 @@ static int __f2fs_write_data_pages(struct address_space *mapping, >>>>>> return ret; >>>>>> skip_write: >>>>>> - wbc->pages_skipped += get_dirty_pages(inode); >>>>>> + wbc->pages_skipped += >>>>>> + mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY) ? >>>>> >>>>> Is there any race condition to get 0, if there's any dirty page? IOWs, it >>>> >>>> Quoted from Jing Xia's explanation: >>>> >>>> [T:writeback] [T:checkpoint] >>> >>> My bad, [1] should be here: >>> >>> bio contains NodeA was plugged in writeback threads >>> >>> 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 >> >> Jaegeuk, >> >> checkpoint() will stuck on waiting NodeA to writeback. >> >>>> - inode_dec_dirty_pages >>> >>>> bio contains NodeA was plugged in writeback threads >>> >>> [1] >>> >>> Thanks, >>> >>>> - 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 >> >> Meanwhile, kworker plugged NodeA, and then loop writebacking inode due to >> inode will be requeued all the time. > > I'm thinking that this will be released after finishing node writes in a > different context. Then, this loop can happen in that time period which > is NOT forever. But, if we want to avoid this temporary loop, we may be
I doubt it may break due to below condition in writeback_sb_inodes():
if (need_resched()) { /* * We're trying to balance between building up a nice * long list of IOs to improve our merge rate, and * getting those IOs out quickly for anyone throttling * in balance_dirty_pages(). cond_resched() doesn't * unplug, so get our IOs out the door before we * give up the CPU. */ if (current->plug) blk_flush_plug(current->plug, false); cond_resched(); }
But as Zhiguo Niu and Jing Xia reported, blk_flush_plug was not executed...
> able to decrease the dirty count before f2fs_update_data_blkaddr().
I guess there is one simple way, we can force flushing plugged bio via blk_flush_plug(current->plug, false) once we skip writeback in writepages() due to sbi->wb_sync_req[DATA/NODE] is valid.
Thoughts?
Thanks,
> >> >> Thanks, >> >>>> >>>>> seems the current condition is just requeuing the inode as dirty, but next >>>>> flushing time will remove it from dirty list. Is this giving too much overheads? >>>> >>>> I prefer to let writeback thread call blk_flush_plug() after skipping >>>> writepages() due to wb_sync_req[DATA/NODE] check condition, thoughts? >>>> >>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c >>>> index 9f754aaef558..b6e1ed73f8f5 100644 >>>> --- a/fs/f2fs/data.c >>>> +++ b/fs/f2fs/data.c >>>> @@ -3087,6 +3087,8 @@ static int f2fs_write_cache_pages(struct address_space *mapping, >>>> /* give a priority to WB_SYNC threads */ >>>> if (atomic_read(&sbi->wb_sync_req[DATA]) && >>>> wbc->sync_mode == WB_SYNC_NONE) { >>>> + if (current->plug) >>>> + blk_flush_plug(current->plug, false); >>>> done = 1; >>>> break; >>>> } >>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c >>>> index 556fcd8457f3..dd9a817d8dab 100644 >>>> --- a/fs/f2fs/node.c >>>> +++ b/fs/f2fs/node.c >>>> @@ -1946,6 +1946,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi, >>>> if (atomic_read(&sbi->wb_sync_req[NODE]) && >>>> wbc->sync_mode == WB_SYNC_NONE) { >>>> done = 1; >>>> + if (current->plug) >>>> + blk_flush_plug(current->plug, false); >>>> break; >>>> } >>>> >>>> >>>> >>>> Thanks, >>>> >>>>> >>>>>> + get_dirty_pages(inode) : 0; >>>>>> trace_f2fs_writepages(mapping->host, wbc, DATA); >>>>>> return 0; >>>>>> } >>>>>> -- >>>>>> 2.28.0 >>>> >>>> >>>> _______________________________________________ >>>> Linux-f2fs-devel mailing list >>>> Linux-f2fs-devel@lists.sourceforge.net >>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel >>>> >>> >>> >>> _______________________________________________ >>> Linux-f2fs-devel mailing list >>> Linux-f2fs-devel@lists.sourceforge.net >>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
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