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

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Date:	Tue, 28 Apr 2015 20:35:36 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 28-04-15 09:57:11, Linus Torvalds wrote:
> 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.

I am still not sure I see the problem here. Let's say we have a
userspace page fault handler which would do mmap(fault_addr, MAP_FIXED),
right?

If we had a racy mmap(NULL, MAP_LOCKED) that could have mapped
fault_addr by the time handler does its work then this is buggy wrt. to
MAP_LOCKED semantic because the fault handler would discard the locked
part. This wouldn't lead to a data loss but still makes MAP_LOCKED usage
buggy IMO.

If the racing thread did mmap(around_fault_addr, MAP_FIXED|MAP_LOCKED)
then it would be broken as well, and even worse I would say, because the
original fault could have been discarded and data lost.

I would expect that user fault handlers would be synchronized with
other mmap activity otherwise I have hard time to see how this can all
have a well defined behavior. Especially when MAP_FIXED is involved.

> 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..

Actually we already allow to mmap(MAP_FIXED) to fail after
discarding an existing mmaped area (see mmap_region and e.g.
security_vm_enough_memory_mm or other failure cases).
-- 
Michal Hocko
SUSE Labs
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