linux-kernel - Re: [PATCH v2 3/5] mm: add static PMD zero page

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Message-ID: <ea55eb30-552a-4fca-83e0-342ec7c98768@redhat.com>
Date: Thu, 17 Jul 2025 13:46:03 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
 Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>,
 Mike Rapoport <rppt@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Michal Hocko <mhocko@...e.com>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
 willy@...radead.org, linux-mm@...ck.org, x86@...nel.org,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org,
 gost.dev@...sung.com, hch@....de, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 3/5] mm: add static PMD zero page

On 17.07.25 12:34, Pankaj Raghav (Samsung) wrote:
>>> Then, we'd only need a config option to allow for that to happen.
>>
>> Something incomplete and very hacky just to give an idea. It would try allocating
>> it if there is actual code running that would need it, and then have it
>> stick around forever.
>>
> Thanks a lot for this David :) I think this is a much better idea and
> reduces the amount code and reuse the existing infrastructure.
> 
> I will try this approach in the next version.
> 
> <snip>
>> +       /*
>> +        * Our raised reference will prevent the shrinker from ever having
>> +        * success -> static.
>> +        */
>> +       if (atomic_read(&huge_zero_folio_is_static))
>> +               return huge_zero_folio;
>> +       /* TODO: memblock allocation if buddy is not up yet? Or Reject that earlier. */
> 
> Do we need memblock allocation? At least the use cases I forsee for
> static pmd zero page are all after the mm is up. So I don't see why we
> need to allocate it via memblock.

Even better!

We might want to detect whether allocation of the huge zeropage failed a 
couple of times and then just give up. Otherwise, each and every user of 
the largest zero folio will keep allocating it.

-- 
Cheers,

David / dhildenb