Messages in this thread Patch in this message | | | Date | Thu, 18 Nov 2021 14:46:07 +0800 | Subject | Re: [PATCH Vx 1/1] f2fs: Avoid deadlock between writeback and checkpoint | From | Chao Yu <> |
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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] - 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 bio contains NodeA was plugged in writeback threads - 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
> 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
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