Re: [RFC] mm: Add MADV_NONZERO to skip redundant zeroing in MADV_DONTNEED

[David Hildenbrand <david@redhat.com>]

On 04.06.25 03:49, Harry Yoo wrote:
> On Wed, Jun 04, 2025 at 06:31:59AM +0530, Ayaan Mirza Baig wrote:
>> Hi Andrew,
>>
>> I've been looking at MADV_DONTNEED and noticed we always zero pages, even
>> when
>> it might not be necessary for cases like:
>>
>> - Media buffers (video/audio processing)
>> - Temporary compute workloads
>> - Pre-zeroed memory pools
> 
> Hi Ayaan,
> 
> I don't think this "skipping zeroing" is limited to MADV_DONTNEED at all?
> 
> We don't zero pages during zapping (unless init_on_free is enabled),
> but zero pages when madvise(MADV_DONTNEED)'d area is accessed again
> (for anonymous mappings), because that's how faulting anonymous mappings
> works.
> 
> And if you're suggesting that you want to skip zeroing (on some conditions)
> when faulting an anonymous mapping, we can't do that because applications
> rely on the guarantee that such pages are zeroed.

Yeah, no way we're adding something like that.

What we discussed last years at plumbers IIRC in Rik's session was 
having a per-process cache of zapped pages. Essentially, during 
MADV_DONTNEED we could move the pages to that cache, and on re-access 
(if the process opted in) we could grab one of these pages from the 
per-process cache without zeroing them out. On memory pressure we would 
flush the cache.

There are some challenges (e.g., how to keep these pages 
migratable/swappable etc).

-- 
Cheers,

David / dhildenb