linux-kernel - Re: [PATCHv2 2/2] mm/page_poisoning.c: Allow for zero poisoning

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Message-ID: <56ABDB4A.2040709@redhat.com>
Date:	Fri, 29 Jan 2016 13:36:10 -0800
From:	Laura Abbott <labbott@...hat.com>
To:	Pavel Machek <pavel@...x.de>,
	Laura Abbott <labbott@...oraproject.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Michal Hocko <mhocko@...e.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	Kees Cook <keescook@...omium.org>, linux-pm@...r.kernel.org
Subject: Re: [PATCHv2 2/2] mm/page_poisoning.c: Allow for zero poisoning

On 01/29/2016 02:45 AM, Pavel Machek wrote:
> Hi!
>
>> By default, page poisoning uses a poison value (0xaa) on free. If this
>> is changed to 0, the page is not only sanitized but zeroing on alloc
>> with __GFP_ZERO can be skipped as well. The tradeoff is that detecting
>> corruption from the poisoning is harder to detect. This feature also
>> cannot be used with hibernation since pages are not guaranteed to be
>> zeroed after hibernation.
>
> So... this makes kernel harder to debug for performance advantage...?
> If so.. how big is the performance advantage?
> 									Pavel
>

The performance advantage really depends on the benchmark you are running.
It was pointed out this may help some unknown amount with merging pages
in VMs since the pages are now identical and can be merged. The debugging
is also only slightly more difficult. With the non-zero poisoning value
it's easier to see that a crash was caused by triggering the poison vs.
just some random NULL pointer.

As as been pointed out, this help text could use some updating so I'll
clarify this more.

Thanks,
Laura