Re: [PATCH] mm: migrate: buffer_migrate_folio_norefs() fallback migrate not uptodate pages

[Zhihao Cheng]

在 2022/8/25 18:57, Jan Kara 写道:
> On Thu 25-08-22 16:01:46, Zhihao Cheng wrote:
> > From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> >
> > Recently we notice that ext4 filesystem occasionally fail to read
> > metadata from disk and report error message, but the disk and block
> > layer looks fine. After analyse, we lockon commit 88dbcbb3a484
> > ("blkdev: avoid migration stalls for blkdev pages"). It provide a
> > migration method for the bdev, we could move page that has buffers
> > without extra users now, but it will lock the buffers on the page, which
> > breaks a lot of current filesystem's fragile metadata read operations,
> > like ll_rw_block() for common usage and ext4_read_bh_lock() for ext4,
> > these helpers just trylock the buffer and skip submit IO if it lock
> > failed, many callers just wait_on_buffer() and conclude IO error if the
> > buffer is not uptodate after buffer unlocked.
> >
> > This issue could be easily reproduced by add some delay just after
> > buffer_migrate_lock_buffers() in __buffer_migrate_folio() and do
> > fsstress on ext4 filesystem.
> >
> >    EXT4-fs error (device pmem1): __ext4_find_entry:1658: inode #73193:
> >    comm fsstress: reading directory lblock 0
> >    EXT4-fs error (device pmem1): __ext4_find_entry:1658: inode #75334:
> >    comm fsstress: reading directory lblock 0
> >
> > Something like ll_rw_block() should be used carefully and seems could
> > only be safely used for the readahead case. So the best way is to fix
> > the read operations in filesystem in the long run, but now let us avoid
> > this issue first. This patch avoid this issue by fallback to migrate
> > pages that are not uotodate like fallback_migrate_folio(), those pages
> > that has buffers may probably do read operation soon.
> >
> > Fixes: 88dbcbb3a484 ("blkdev: avoid migration stalls for blkdev pages")
> > Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> > Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
>
> Thanks for the analysis and the fix! As you noted above this is actually a
> bug in the filesystems that they assume that locked buffer means it is
> under IO.  Usually that is the case but there are other places that lock
> the buffer without doing IO. Page migration is one of them, jbd2 machinery
> another one, there may be others.
>
> So I think this really ought to be fixed in filesystems instead of papering
> over the bug in the migration code. I agree this is more work but we will
> reduce the technical debt, not increase it :). Honestly, ll_rw_block()
> should just die. It is actively dangerous to use. Instead we should have
> one call for readahead of bhs and the rest should be converted to
> submit_bh() or similar calls. There are like 25 callers remaining so it
> won't be even that hard.
>
> And then we have the same buggy code as in ll_rw_block() copied to
> ext4_bread_batch() (ext4_read_bh_lock() in particular) so that needs to be
> fixed as well...
>
> 								Honza

You are right, Jan. I totally agree with you. It seems that I shouldn't 
have been lazy.