Re: [PATCH -v5 0/9] migrate_pages(): batch TLB flushing

[Hugh Dickins <hughd@google.com>]

On Tue, 28 Feb 2023, Huang, Ying wrote:
> Jan Kara <jack@suse.cz> writes:
> > On Fri 17-02-23 13:47:48, Hugh Dickins wrote:
> >> 
> >> Cc'ing Jan Kara, who knows buffer_migrate_folio_norefs() and jbd2
> >> very well, and I hope can assure us that there is an understandable
> >> deadlock here, from holding several random folio locks, then trying
> >> to lock buffers.  Cc'ing fsdevel, because there's a risk that mm
> >> folk think something is safe, when it's not sufficient to cope with
> >> the diversity of filesystems.  I hope nothing more than the below is
> >> needed (and I've had no other problems with the patchset: good job),
> >> but cannot be sure.
> >
> > I suspect it can indeed be caused by the presence of the loop device as
> > Huang Ying has suggested. What filesystems using buffer_heads do is a
> > pattern like:
> >
> > bh = page_buffers(loop device page cache page);
> > lock_buffer(bh);
> > submit_bh(bh);
> > - now on loop dev this ends up doing:
> >   lo_write_bvec()
> >     vfs_iter_write()
> >       ...
> >       folio_lock(backing file folio);
> >
> > So if migration code holds "backing file folio" lock and at the same time
> > waits for 'bh' lock (while trying to migrate loop device page cache page), it
> > is a deadlock.
> >
> > Proposed solution of never waiting for locks in batched mode looks like a
> > sensible one to me...
> 
> Thank you very much for detail explanation!

Yes, thanks a lot, Jan, for elucidating the deadlocks.

I was running with the 1/3,2/3,3/3 series for 48 hours on two machines
at the weekend, and hit no problems with all of them on.

I did try to review them this evening, but there's too much for me to
take in there to give an Acked-by: but I'll ask a couple of questions.

Hugh