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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andreas Gruenbacher <agruenba@redhat.com>
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: Tue, 3 Aug 2021 12:45:04 -0700	[thread overview]
Message-ID: <CAHk-=wj+_Y7NQ-NhhE0jk52c9ZB0VJbO1AjtMJFB8wP=PO+bdw@mail.gmail.com> (raw)
In-Reply-To: <20210803191818.993968-1-agruenba@redhat.com>

On Tue, Aug 3, 2021 at 12:18 PM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>
>
> With this patch queue, fstest generic/208 (aio-dio-invalidate-failure.c)
> endlessly spins in gfs2_file_direct_write.  It looks as if there's a bug
> in get_user_pages_fast when called with FOLL_FAST_ONLY:
>
>  (1) The test case performs an aio write into a 32 MB buffer.
>
>  (2) The buffer is initially not in memory, so when iomap_dio_rw() ->
>      ... -> bio_iov_iter_get_pages() is called with the iter->noio flag
>      set, we get to get_user_pages_fast() with FOLL_FAST_ONLY set.
>      get_user_pages_fast() returns 0, which causes
>      bio_iov_iter_get_pages to return -EFAULT.
>
>  (3) Then gfs2_file_direct_write faults in the entire buffer with
>      fault_in_iov_iter_readable(), which succeeds.
>
>  (4) With the buffer in memory, we retry the iomap_dio_rw() ->
>      ... -> bio_iov_iter_get_pages() -> ... -> get_user_pages_fast().
>      This should succeed now, but get_user_pages_fast() still returns 0.

Hmm. Have you tried to figure out why that "still returns 0" happens?

One option - for debugging only - would be to introduce a new flag to
get_user_pages_fast() tyhat says "print out reason if failed" and make
the retry (but not the original one) have that flag set.

There are a couple of things of note when it comes to "get_user_pages_fast()":

 (a) some architectures don't even enable it

 (b) it can be very picky about the page table contents, and wants the
accessed bit to already be set (or the dirty bit, in the case of a
write).

but (a) shouldn't be an issue on any common platform and (b) shouldn't
be an issue with  fault_in_iov_iter_readable() that actually does a
__get_user() so it will access through the page tables.

(It might be more of an issue with fault_in_iov_iter_writable() due to
walking the page tables by hand - if we don't do the proper
access/dirty setting, I could see get_user_pages_fast() failing).

Anyway, for reason (a) I do think that eventually we should probably
introduce FOLL_NOFAULT, and allow the full "slow" page table walk -
just not calling down to handle_mm_fault() if it fails.

But (a) should be a non-issue in your test environment, and so it
would be interesting to hear what it is that fails. Because scanning
through the patches, they all _look_ fine to me (apart from the one
comment about return values, which is more about being consistent with
copy_to/from_user() and making the code simpler - not about
correctness)

                       Linus

  parent reply	other threads:[~2021-08-03 19:45 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 ` Linus Torvalds [this message]
2021-08-16 19:14   ` [PATCH v5 00/12] gfs2: Fix mmap + page fault deadlocks 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

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