Re: [RFC PATCH 4/4] mm: Add PG_zero support

[David Hildenbrand]

On 24.04.20 02:41, Matthew Wilcox wrote:
> On Thu, Apr 23, 2020 at 05:37:00PM -0700, Andrew Morton wrote:
>> On Wed, 22 Apr 2020 16:09:00 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
>>> Heh, I was quite sure that this is not the first time background zeroing is
>>> proposed, so I went to google for it... and found that one BSD kernel actually
>>> removed this functionality in 2016 [1] and this was one of the reasons.
>>>
>>> [1]
>>> https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/afd2da4dc9056ea79cdf15e8a9386a3d3998f33e
>>
>> Interesting.
>>
>> However this:
>>
>>   - Pre-zeroing a page leads to a cold-cache case on-use, forcing the fault
>>     source (e.g. a userland program) to actually get the data from main
>>     memory in its likely immediate use of the faulted page, reducing
>>     performance.
>>
>> implies that BSD was zeroing with non-temporal stores which bypass the
>> CPU cache.  And which presumably invalidate any part of the target
>> memory which was already in cache.  We wouldn't do it that way so
>> perhaps the results would differ.
> 
> Or just that the page was zeroed far enough in advance that it fell out
> of cache naturally.
> 
> I know Arjan looked at zeroing on free instead of zeroing on alloc,
> and that didn't get merged (or even submitted afaik), so presumably the
> results weren't good.

We do have INIT_ON_FREE_DEFAULT_ON

via

commit 6471384af2a6530696fc0203bafe4de41a23c9ef
Author: Alexander Potapenko <glider@xxxxxxxxxx>
Date:   Thu Jul 11 20:59:19 2019 -0700

    mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

which seems to do exactly that (although for a different use case)

-- 
Thanks,

David / dhildenb