linux-kernel - Re: [PATCH 06/32] sched: Add task_struct->faults_disabled

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Date:   Tue, 23 May 2023 12:49:06 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        "Darrick J . Wong" <djwong@...nel.org>, dhowells@...hat.com,
        Andreas Gruenbacher <agruenba@...hat.com>,
        cluster-devel@...hat.com, Bob Peterson <rpeterso@...hat.com>
Subject: Re: [PATCH 06/32] sched: Add task_struct->faults_disabled_mapping

> > No, that's definitely handled (and you can see it in the code I linked),
> > and I wrote a torture test for fstests as well.
> 
> 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.

Oof, that sounds a bit sketchy. What happens if the dio call passes in
an address from the same address space? What happens if we race with the
pages we faulted in being evicted?

> 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?

More tests more good.

So if we want to lift this scheme to the VFS layer, we'd start by
replacing the lock you added (grepping for it, the name escapes me) with
a different type of lock - two_state_shared_lock in my code, it's like a
rw lock except writers don't exclude other writers. That way the DIO
path can use it without singlethreading writes to a single file.