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Date: Sat, 7 Apr 2012 19:33:18 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Hugh Dickins <hughd@google.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Roland Dreier <roland@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem
Message-ID: <20120407173318.GA5076@redhat.com>
References: <20120402153631.5101.44091.stgit@zurg> <20120403143752.GA5150@redhat.com> <4F7C1B67.6030300@openvz.org> <20120404154148.GA7105@redhat.com> <4F7D5859.5050106@openvz.org> <alpine.LSU.2.00.1204062104090.4297@eggly.anvils>
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On 04/06, Hugh Dickins wrote:
>
> I've long detested that behaviour of GUP write,force, and my strong
> preference would be not to layer more strangeness upon strangeness,
> but limit the damage by making GUP write,force fail in that case,
> instead of inserting a PageAnon page into a VM_SHARED mapping.
>
> I think it's unlikely that it will cause a regression in real life
> (it already fails if you did not open the mmap'ed file for writing),

Yes, and this is what looks confusing to me. Assuming I understand
you (and the code) correctly ;)

If we have a (PROT_READ, MAP_SHARED) file mapping, then FOLL_FORCE
works depending on "file->f_mode & FMODE_WRITE".

Afaics, because do_mmap_pgoff(MAP_SHARED) clears VM_MAYWRITE if
!FMODE_WRITE, and gup(FORCE) checks "vma->vm_flags & VM_MAYWRITE"
before follow_page/etc.

OTOH, if the file was opened without FMODE_WRITE, then I do not
really understand how (PROT_READ, MAP_SHARED) differs from
(PROT_READ, MAP_PRIVATE). However, in the latter case FOLL_FORCE
works, VM_MAYWRITE was not cleared.

Speaking of the difference above, I'd wish I could understand
what VM_MAYSHARE actually means except "MAP_SHARED was used".

Oleg.