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

[Vlastimil Babka]

On 4/24/20 9:28 AM, David Hildenbrand wrote:
> 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.

Agreed.

>> 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)

Yeah, except the security use case wants to do that immediately, while the
proposal here is a background thread.