linux-kernel - Re: can't oom-kill zap the victim's memory?

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Message-ID: <CA+55aFz1HFLVNeAaOWK=-Wyq8FF5bhWpWk8Dnwpa-8vD2k+b+A@mail.gmail.com>
Date:	Tue, 6 Oct 2015 09:55:33 +0100
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Kyle Walker <kwalker@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stanislav Kozina <skozina@...hat.com>
Subject: Re: can't oom-kill zap the victim's memory?

On Tue, Oct 6, 2015 at 9:49 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The basic fact remains: kernel allocations are so important that
> rather than fail, you should kill user space. Only kernel allocations
> that *explicitly* know that they have fallback code should fail, and
> they should just do the __GFP_NORETRY.

To be clear: "big" orders (I forget if the limit is at order-3 or
order-4) do fail much more aggressively. But no, we do not limit retry
to just order-0, because even small kmalloc sizes tend to often do
order-1 or order-2 just because of memory packing issues (ie trying to
pack into a single page wastes too much memory if the allocation sizes
don't come out right).

So no, order-0 isn't special. 1/2 are rather important too.

[ Checking /proc/slabinfo: it looks like several slabs are order-3,
for things like files_cache, signal_cache and sighand_cache for me at
least. So I think it's up to order-3 that we basically need to
consider "we'll need to shrink user space aggressively unless we have
an explicit fallback for the allocation" ]

            Linus
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