On Wed 13-09-23 10:42:21, Christian Brauner wrote:
> [+Cc Jan]
Thanks!
> On Tue, Sep 12, 2023 at 08:20:43AM -0600, Chunhai Guo wrote:
> > I am encountering a deadlock issue as shown below. There is a commit
> > 344150999b7f ("f2fs: fix to avoid potential deadlock") can fix this issue.
> > However, from log analysis, it appears that this is more likely a fake
> > progress issue similar to commit 68f4c6eba70d ("fs-writeback:
> > writeback_sb_inodes: Recalculate 'wrote' according skipped pages"). In each
> > writeback iteration, nothing is written, while writeback_sb_inodes()
> > increases 'total_wrote' each time, causing an infinite loop. This patch
> > fixes this issue by not increasing 'total_wrote' when nothing is written.
> >
> > wb_writeback fsync (inode-Y)
> > blk_start_plug(&plug)
> > for (;;) {
> > iter i-1: some reqs with page-X added into plug->mq_list // f2fs node page-X with PG_writeback
> > filemap_fdatawrite
> > __filemap_fdatawrite_range // write inode-Y with sync_mode WB_SYNC_ALL
> > do_writepages
> > f2fs_write_data_pages
> > __f2fs_write_data_pages // wb_sync_req[DATA]++ for WB_SYNC_ALL
> > f2fs_write_cache_pages
> > f2fs_write_single_data_page
> > f2fs_do_write_data_page
> > f2fs_outplace_write_data
> > f2fs_update_data_blkaddr
> > f2fs_wait_on_page_writeback
> > wait_on_page_writeback // wait for f2fs node page-X
> > iter i:
> > progress = __writeback_inodes_wb(wb, work)
> > . writeback_sb_inodes
> > . __writeback_single_inode // write inode-Y with sync_mode WB_SYNC_NONE
> > . . do_writepages
> > . . f2fs_write_data_pages
> > . . . __f2fs_write_data_pages // skip writepages due to (wb_sync_req[DATA]>0)
> > . . . wbc->pages_skipped += get_dirty_pages(inode) // wbc->pages_skipped = 1
> > . if (!(inode->i_state & I_DIRTY_ALL)) // i_state = I_SYNC | I_SYNC_QUEUED
> > . total_wrote++; // total_wrote = 1
> > . requeue_inode // requeue inode-Y to wb->b_dirty queue due to non-zero pages_skipped
> > if (progress) // progress = 1
> > continue;
> > iter i+1:
> > queue_io
> > // similar process with iter i, infinite for-loop !
> > }
> > blk_finish_plug(&plug) // flush plug won't be called
> >
> > Signed-off-by: Chunhai Guo <guochunhai@xxxxxxxx>
Thanks for the patch but did you test this patch fixed your deadlock?
Because the patch seems like a noop to me. Look:
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 969ce991b0b0..54cdee906be9 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -1820,6 +1820,7 @@ static long writeback_sb_inodes(struct super_block *sb,
> > struct inode *inode = wb_inode(wb->b_io.prev);
> > struct bdi_writeback *tmp_wb;
> > long wrote;
> > + bool is_dirty_before;
> >
> > if (inode->i_sb != sb) {
> > if (work->sb) {
> > @@ -1881,6 +1882,7 @@ static long writeback_sb_inodes(struct super_block *sb,
> > continue;
> > }
> > inode->i_state |= I_SYNC;
> > + is_dirty_before = inode->i_state & I_DIRTY_ALL;
is_dirty_before is going to be set if there's anything dirty - inode, page,
timestamp. So it can be unset only if there are no dirty pages, in which
case there are no pages that can be skipped during page writeback, which means
that requeue_inode() will go and remove inode from b_io/b_dirty lists and
it will not participate in writeback anymore.
So I don't see how this patch can be helping anything... Please correct me
if I'm missing anything.
Honza
> > wbc_attach_and_unlock_inode(&wbc, inode);
> >
> > write_chunk = writeback_chunk_size(wb, work);
> > @@ -1918,7 +1920,7 @@ static long writeback_sb_inodes(struct super_block *sb,
> > */
> > tmp_wb = inode_to_wb_and_lock_list(inode);
> > spin_lock(&inode->i_lock);
> > - if (!(inode->i_state & I_DIRTY_ALL))
> > + if (!(inode->i_state & I_DIRTY_ALL) && is_dirty_before)
> > total_wrote++;
> > requeue_inode(inode, tmp_wb, &wbc);
> > inode_sync_complete(inode);
> > --
> > 2.25.1
> >
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
