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Date: Tue, 28 Apr 2015 09:57:11 -0700
Message-ID: <CA+55aFydkG-BgZzry5DrTzueVh9VvEcVJdLV8iOyUphQk=0vpw@mail.gmail.com>
Subject: Re: Should mmap MAP_LOCKED fail if mm_poppulate fails?
From: Linus Torvalds <torvalds@linux-foundation.org>
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To: Michal Hocko <mhocko@suse.cz>
Cc: linux-mm <linux-mm@kvack.org>, Cyril Hrubis <chrubis@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>, Michel Lespinasse <walken@google.com>, Rik van Riel <riel@redhat.com>, Michael Kerrisk <mtk.manpages@gmail.com>, LKML <linux-kernel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>

On Tue, Apr 28, 2015 at 9:43 AM, Michal Hocko <mhocko@suse.cz> wrote:
>
> Hmm, no other thread has the address from the current mmap call except
> for MAP_FIXED (more on that below).

With things like opportunistic SIGSEGV handlers that map/unmap things
as the user takes faults, that's actually not at all guaranteed.

Yeah, it's unusual, but I've seen it, with threaded applications where
people play games with user-space memory management, and do "demand
allocation" with mmap() in response to signals.

Admittedly we already do bad things in mmap(MAP_FIXED) for that case,
since we dropped the vm lock. But at least it shouldn't be any worse
than a thread speculatively touching the pages..

                      Linus

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