linux-kernel - Re: [PATCH 0/5] add STATIC_PMD_ZERO

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Message-ID: <76a48d80-7eb0-4196-972d-ecdcbd4ae709@intel.com>
Date: Thu, 12 Jun 2025 14:46:34 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Pankaj Raghav <p.raghav@...sung.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....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>, Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jens Axboe <axboe@...nel.dk>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, willy@...radead.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
Subject: Re: [PATCH 0/5] add STATIC_PMD_ZERO_PAGE config option

On 6/12/25 13:36, Pankaj Raghav (Samsung) wrote:
> On Thu, Jun 12, 2025 at 06:50:07AM -0700, Dave Hansen wrote:
>> On 6/12/25 03:50, Pankaj Raghav wrote:
>>> But to use huge_zero_folio, we need to pass a mm struct and the
>>> put_folio needs to be called in the destructor. This makes sense for
>>> systems that have memory constraints but for bigger servers, it does not
>>> matter if the PMD size is reasonable (like in x86).
>>
>> So, what's the problem with calling a destructor?
>>
>> In your last patch, surely bio_add_folio() can put the page/folio when
>> it's done. Is the real problem that you don't want to call zero page
>> specific code at bio teardown?
> 
> Yeah, it feels like a lot of code on the caller just to use a zero page.
> It would be nice just to have a call similar to ZERO_PAGE() in these
> subsystems where we can have guarantee of getting huge zero page.
> 
> Apart from that, these are the following problems if we use
> mm_get_huge_zero_folio() at the moment:
> 
> - We might end up allocating 512MB PMD on ARM systems with 64k base page
>   size, which is undesirable. With the patch series posted, we will only
>   enable the static huge page for sane architectures and page sizes.

Does *anybody* want the 512MB huge zero page? Maybe it should be an
opt-in at runtime or something.

> - In the current implementation we always call mm_put_huge_zero_folio()
>   in __mmput()[1]. I am not sure if model will work for all subsystems. For
>   example bio completions can be async, i.e, we might need a reference
>   to the zero page even if the process is no longer alive.

The mm is a nice convenient place to stick an mm but there are other
ways to keep an efficient refcount around. For instance, you could just
bump a per-cpu refcount and then have the shrinker sum up all the
refcounts to see if there are any outstanding on the system as a whole.

I understand that the current refcounts are tied to an mm, but you could
either replace the mm-specific ones or add something in parallel for
when there's no mm.