linux-kernel - Re: Should mmap MAP_LOCKED fail if mm

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Date:	Tue, 28 Apr 2015 09:57:11 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm <linux-mm@...ck.org>, Cyril Hrubis <chrubis@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: Should mmap MAP_LOCKED fail if mm_poppulate fails?

On Tue, Apr 28, 2015 at 9:43 AM, Michal Hocko <mhocko@...e.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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