linux-kernel - Re: [PATCH v2 0/5] add static PMD zero page support

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Message-ID: <3ed1e744-5536-4b47-a5ab-66cd300ded67@pankajraghav.com>
Date: Wed, 9 Jul 2025 11:59:00 +0200
From: Pankaj Raghav <kernel@...kajraghav.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.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>, 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 0/5] add static PMD zero page support

Hi Andrew,

>> We already have huge_zero_folio that is allocated on demand, and it will be
>> deallocated by the shrinker if there are no users of it left.
>>
>> At moment, huge_zero_folio infrastructure refcount is tied to the process
>> lifetime that created it. This might not work for bio layer as the completions
>> can be async and the process that created the huge_zero_folio might no
>> longer be alive.
> 
> Can we change that?  Alter the refcounting model so that dropping the
> final reference at interrupt time works as expected?
> 

That is an interesting point. I did not try it. At the moment, we always drop the reference in
__mmput().

Going back to the discussion before this work started, one of the main thing that people wanted was
to use some sort of a **drop in replacement** for ZERO_PAGE that can be bigger than PAGE_SIZE[1].

And, during the RFCs of these patches, one of the feedback I got from David was in big server
systems, 2M (in the case of 4k page size) should not be a problem and we don't need any unnecessary
refcounting for them.

Also when I had a chat with David, he also wants to make changes to the existing mm_huge_zero_folio
infrastructure to get rid of shrinker if possible. So we decided that it is better to have opt-in
static allocation and keep the existing dynamic allocation path.

So that is why I went with this approach of having a static PMD allocation.

I hope this clarifies the motivation a bit.

Let me know if you have more questions.

> And if we were to do this, what sort of benefit might it produce?
> 
>> Add a config option STATIC_PMD_ZERO_PAGE that will always allocate
>> the huge_zero_folio via memblock, and it will never be freed.
[1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/