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Message-ID: <5679943C.1050604@intel.com>
Date: Tue, 22 Dec 2015 10:19:40 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Christoph Lameter <cl@...ux.com>
Cc: kernel-hardening@...ts.openwall.com, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>, Laura Abbott
<laura@...bott.name>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Kees Cook <keescook@...omium.org>
Subject: Re: [RFC][PATCH 6/7] mm: Add Kconfig option for
slab sanitization
On 12/22/2015 10:08 AM, Christoph Lameter wrote:
> On Tue, 22 Dec 2015, Dave Hansen wrote:
>>> Why would you use zeros? The point is just to clear the information right?
>>> The regular poisoning does that.
>>
>> It then allows you to avoid the zeroing at allocation time.
>
> Well much of the code is expecting a zeroed object from the allocator and
> its zeroed at that time. Zeroing makes the object cache hot which is an
> important performance aspect.
Yes, modifying this behavior has a performance impact. It absolutely
needs to be evaluated, and I wouldn't want to speculate too much on how
good or bad any of the choices are.
Just to reiterate, I think we have 3 real choices here:
1. Zero at alloc, only when __GFP_ZERO
(behavior today)
2. Poison at free, also Zero at alloc (when __GFP_ZERO)
(this patch's proposed behavior, also what current poisoning does,
doubles writes)
3. Zero at free, *don't* Zero at alloc (when __GFP_ZERO)
(what I'm suggesting, possibly less perf impact vs. #2)
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