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From: Andreas Gruenbacher <agruenba@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Paul Mackerras <paulus@ozlabs.org>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	cluster-devel <cluster-devel@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	ocfs2-devel@oss.oracle.com, kvm-ppc@vger.kernel.org
Subject: Re: [PATCH v5 00/12] gfs2: Fix mmap + page fault deadlocks
Date: Thu, 19 Aug 2021 23:39:56 +0200	[thread overview]
Message-ID: <CAHc6FU6a8SLmHfMoS7NUDKboWpVEGBKyC46pU_brx3y8crbEXA@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wgumKBhggjyR7Ff6V8VKxaJK1yA-LpWdzZFSqFyqYq0Dw@mail.gmail.com>

On Thu, Aug 19, 2021 at 10:14 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Aug 19, 2021 at 12:41 PM Andreas Gruenbacher
> <agruenba@redhat.com> wrote:
> >
> > Hmm, what if GUP is made to skip VM_IO vmas without adding anything to
> > the pages array? That would match fault_in_iov_iter_writeable, which
> > is modeled after __mm_populate and which skips VM_IO and VM_PFNMAP
> > vmas.
>
> I don't understand what you mean.. GUP already skips VM_IO (and
> VM_PFNMAP) pages. It just returns EFAULT.
>
> We could make it return another error. We already have DAX and
> FOLL_LONGTERM returning -EOPNOTSUPP.
>
> Of course, I think some code ends up always just returning "number of
> pages looked up" and might return 0 for "no pages" rather than the
> error for the first page.
>
> So we may end up having interfaces that then lose that explanation
> error code, but I didn't check.
>
> But we couldn't make it just say "skip them and try later addresses",
> if that is what you meant. THAT makes no sense - that would just make
> GUP look up some other address than what was asked for.

get_user_pages has a start and a nr_pages argument, which specifies an
address range from start to start + nr_pages * PAGE_SIZE. If pages !=
NULL, it adds a pointer to that array for each PAGE_SIZE subpage. I
was thinking of skipping over VM_IO vmas in that process, so when the
range starts in a mappable vma, runs into a VM_IO vma, and ends in a
mappable vma, the pages in the pages array would be discontiguous;
they would only cover the mappable vmas. But that would make it
difficult to make sense of what's in the pages array. So scratch that
idea.

> > > I also do still think that even regardless of that, we want to just
> > > add a FOLL_NOFAULT flag that just disables calling handle_mm_fault(),
> > > and then you can use the regular get_user_pages().
> > >
> > > That at least gives us the full _normal_ page handling stuff.
> >
> > And it does fix the generic/208 failure.
>
> Good. So I think the approach is usable, even if we might have corner
> cases left.
>
> So I think the remaining issue is exactly things like VM_IO and
> VM_PFNMAP. Do the fstests have test-cases for things like this? It
> _is_ quite specialized, it might be a good idea to have that.
>
> Of course, doing direct-IO from special memory regions with zerocopy
> might be something special people actually want to do. But I think
> we've had that VM_IO flag testing there basically forever, so I don't
> think it has ever worked (for some definition of "ever").

The v6 patch queue should handle those cases acceptably well for now,
but I don't think we have tests covering that at all.

Thanks,
Andreas


      reply	other threads:[~2021-08-19 21:40 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-03 19:18 Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 01/12] iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 02/12] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 03/12] Turn fault_in_pages_{readable,writeable} into fault_in_{readable,writeable} Andreas Gruenbacher
2021-08-03 19:43   ` Linus Torvalds
2021-08-03 20:57   ` Al Viro
2021-08-03 21:38     ` Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 04/12] Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 05/12] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-08-04 15:24   ` Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 06/12] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 07/12] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 08/12] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 09/12] iomap: Support restarting direct I/O requests after user copy failures Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 10/12] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 11/12] iov_iter: Introduce noio flag to disable page faults Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 12/12] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-08-03 19:45 ` [PATCH v5 00/12] gfs2: Fix mmap + page fault deadlocks Linus Torvalds
2021-08-16 19:14   ` Andreas Gruenbacher
2021-08-18 21:49     ` Linus Torvalds
2021-08-19 19:40       ` Andreas Gruenbacher
2021-08-19 20:14         ` Linus Torvalds
2021-08-19 21:39           ` Andreas Gruenbacher [this message]

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