linux-kernel - Re: suppress page allocation failure warnings from sys

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Message-Id: <20120328190211.4ac8a653.akpm@linux-foundation.org>
Date:	Wed, 28 Mar 2012 19:02:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>
Cc:	Joe Perches <joe@...ches.com>, Dave Chinner <david@...morbit.com>,
	viro@...iv.linux.org.uk,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: suppress page allocation failure warnings from sys_listxattr

On Wed, 28 Mar 2012 21:50:59 -0400 Dave Jones <davej@...hat.com> wrote:

> On Wed, Mar 28, 2012 at 06:46:02PM -0700, Andrew Morton wrote:
> 
>  > Could.  There was some discussion last year and implementations were
>  > tossed around.
>  > 
>  > I'm a bit apprehensive - kernel code is supposed to be robust, and
>  > large allocations are not robust and vmalloc() is crappy.
> 
> Can you expand on crappy ?

It's expensive on a per-call basis and can end up failing due to
internal fragmentation of vmalloc()'s virtually-addressed arena.  I
don't think I've ever seen a report of anyone getting a vmalloc()
failure due to the fragmentation issue, so it's largely theoretical. 
But of course, the more we use it (especially for long-lived
allocations), the greater the risk becomes.  Mainly to 32-bit machines,
I assume.

> Also, what happens if something allocates
> and sits on a bunch of vmalloc'd memory ? would we start seeing oom kills ?

vmalloc() would fail.

> (thinking of the context of my fuzzing tool where a bunch of instances could
>  feasibly call these syscalls and not sit on huge amounts per thread, but
>  collectively...  I'm wondering if it could be provoked into killing
>  processes I don't own)

umm, if you wanted to deliberately trigger a vmalloc() failure then I
guess a good approach would be to locate a vmalloc() site which can
persist beyond the syscall (modprobe is a good one!) then exercise it
in a way so that there are no N-byte holes left in the arena, then
trigger an N-byte vmalloc().
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