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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: Wed, 18 Aug 2021 14:49:28 -0700	[thread overview]
Message-ID: <CAHk-=whBCm3G5yibbvQsTn00fA16a688NTU_geQV158DnRy+bQ@mail.gmail.com> (raw)
In-Reply-To: <CAHc6FU6H5q20qiQ5FX1726i0FJHyh=Y46huWkCBZTR3sk+3Dhg@mail.gmail.com>

[ Sorry for the delay, I was on the road and this fell through the cracks ]

On Mon, Aug 16, 2021 at 12:14 PM Andreas Gruenbacher
<agruenba@redhat.com> wrote:
>
> On Tue, Aug 3, 2021 at 9:45 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Hmm. Have you tried to figure out why that "still returns 0" happens?
>
> The call stack is:
>
> gup_pte_range
> gup_pmd_range
> gup_pud_range
> gup_p4d_range
> gup_pgd_range
> lockless_pages_from_mm
> internal_get_user_pages_fast
> get_user_pages_fast
> iov_iter_get_pages
> __bio_iov_iter_get_pages
> bio_iov_iter_get_pages
> iomap_dio_bio_actor
> iomap_dio_actor
> iomap_apply
> iomap_dio_rw
> gfs2_file_direct_write
>
> In gup_pte_range, pte_special(pte) is true and so we return 0.

Ok, so that is indeed something that the fast-case can't handle,
because some of the special code wants to have the mm_lock so that it
can look at the vma flags (eg "vm_normal_page()" and friends.

That said, some of these cases even the full GUP won't ever handle,
simply because a mapping doesn't necessarily even _have_ a 'struct
page' associated with it if it's a VM_IO mapping.

So it turns out that you can't just always do
fault_in_iov_iter_readable() and then assume that you can do
iov_iter_get_pages() and repeat until successful.

We could certainly make get_user_pages_fast() handle a few more cases,
but I get the feeling that we need to have separate error cases for
EFAULT - no page exists - and the "page exists, but cannot be mapped
as a 'struct page'" case.

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.

                   Linus

  reply	other threads:[~2021-08-18 21:49 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 [this message]
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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