Re: [Cluster-devel] [PATCH 06/32] sched: Add task_struct->faults_disabled_mapping

Andreas Grünbacher
2023-05-26T00:06:29Z

Am Fr., 26. Mai 2023 um 01:20 Uhr schrieb Kent Overstreet
<kent.overstreet@linux.dev>:
> On Fri, May 26, 2023 at 12:25:31AM +0200, Andreas Grünbacher wrote:
> > Am Di., 23. Mai 2023 um 18:28 Uhr schrieb Christoph Hellwig <hch@infradead.org>:
> > > On Tue, May 23, 2023 at 03:34:31PM +0200, Jan Kara wrote:
> > > > I've checked the code and AFAICT it is all indeed handled. BTW, I've now
> > > > remembered that GFS2 has dealt with the same deadlocks - b01b2d72da25
> > > > ("gfs2: Fix mmap + page fault deadlocks for direct I/O") - in a different
> > > > way (by prefaulting pages from the iter before grabbing the problematic
> > > > lock and then disabling page faults for the iomap_dio_rw() call). I guess
> > > > we should somehow unify these schemes so that we don't have two mechanisms
> > > > for avoiding exactly the same deadlock. Adding GFS2 guys to CC.
> > > >
> > > > Also good that you've written a fstest for this, that is definitely a useful
> > > > addition, although I suspect GFS2 guys added a test for this not so long
> > > > ago when testing their stuff. Maybe they have a pointer handy?
> > >
> > > generic/708 is the btrfs version of this.
> > >
> > > But I think all of the file systems that have this deadlock are actually
> > > fundamentally broken because they have a mess up locking hierarchy
> > > where page faults take the same lock that is held over the the direct I/
> > > operation. And the right thing is to fix this. I have work in progress
> > > for btrfs, and something similar should apply to gfs2, with the added
> > > complication that it probably means a revision to their network
> > > protocol.
> >
> > We do disable page faults, and there can be deadlocks in page fault
> > handlers while no page faults are allowed.
> >
> > I'm roughly aware of the locking hierarchy that other filesystems use,
> > and that's something we want to avoid because of two reasons: (1) it
> > would be an incompatible change, and (2) we want to avoid cluster-wide
> > locking operations as much as possible because they are very slow.
> >
> > These kinds of locking conflicts are so rare in practice that the
> > theoretical inefficiency of having to retry the operation doesn't
> > matter.
>
> Would you be willing to expand on that? I'm wondering if this would
> simplify things for gfs2, but you mention locking heirarchy being an
> incompatible change - how does that work?

Oh, it's just that gfs2 uses one dlm lock per inode to control access
to that inode. In the code, this is called the "inode glock" ---
glocks being an abstraction above dlm locks --- but it boils down to
dlm locks in the end. An additional layer of locking will only work
correctly if all cluster nodes use the new locks consistently, so old
cluster nodes will become incompatible. Those kinds of changes are
hard.

But the additional lock taking would also hurt performance, forever,
and I'd really like to avoid taking that hit.

It may not be obvious to everyone, but allowing page faults during
reads and writes (i.e., while holding dlm locks) can lead to
distributed deadlocks. We cannot just pretend to be a local
filesystem.

Thanks,
Andreas

> > > I'm absolutely not in favour to add workarounds for thes kind of locking
> > > problems to the core kernel. I already feel bad for allowing the
> > > small workaround in iomap for btrfs, as just fixing the locking back
> > > then would have avoid massive ratholing.
> >
> > Please let me know when those btrfs changes are in a presentable shape ...
>
> I would also be curious to know what btrfs needs and what the approach
> is there.