Re: suppress page allocation failure warnings from sys_listxattr

From: Andrew Morton
Date: Wed Mar 28 2012 - 21:58:26 EST

On Wed, 28 Mar 2012 21:50:59 -0400 Dave Jones <davej@xxxxxxxxxx> 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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