linux-kernel - Re: [PATCH v1 1/2] mm/huge_memory: introduce uniform_split_unmapped_folio_to_zero

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4C3B115F-5559-430A-A240-A6A291819818@nvidia.com>
Date: Sun, 16 Nov 2025 22:21:26 -0500
From: Zi Yan <ziy@...dia.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Matthew Wilcox <willy@...radead.org>, Jiaqi Yan <jiaqiyan@...gle.com>,
 nao.horiguchi@...il.com, linmiaohe@...wei.com, david@...hat.com,
 lorenzo.stoakes@...cle.com, william.roche@...cle.com, tony.luck@...el.com,
 wangkefeng.wang@...wei.com, jane.chu@...cle.com, akpm@...ux-foundation.org,
 osalvador@...e.de, muchun.song@...ux.dev, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Brendan Jackman <jackmanb@...gle.com>,
 Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v1 1/2] mm/huge_memory: introduce
 uniform_split_unmapped_folio_to_zero_order

On 16 Nov 2025, at 22:15, Harry Yoo wrote:

> On Sun, Nov 16, 2025 at 11:51:14AM +0000, Matthew Wilcox wrote:
>> On Sun, Nov 16, 2025 at 01:47:20AM +0000, Jiaqi Yan wrote:
>>> Introduce uniform_split_unmapped_folio_to_zero_order, a wrapper
>>> to the existing __split_unmapped_folio. Caller can use it to
>>> uniformly split an unmapped high-order folio into 0-order folios.
>>
>> Please don't make this function exist.  I appreciate what you're trying
>> to do, but let's try to do it differently?
>>
>> When we have struct folio separately allocated from struct page,
>> splitting a folio will mean allocating new struct folios for every
>> new folio created.  I anticipate an order-0 folio will be about 80 or
>> 96 bytes.  So if we create 512 * 512 folios in a single go, that'll be
>> an allocation of 20MB.
>>
>> This is why I asked Zi Yan to create the asymmetrical folio split, so we
>> only end up creating log() of this.  In the case of a single hwpoison page
>> in an order-18 hugetlb, that'd be 19 allocations totallying 1520 bytes.
>
> Oh god, I completely overlooked this aspect when discussing this with Jiaqi.
> Thanks for raising this concern.
>
>> But since we're only doing this on free, we won't need to do folio
>> allocations at all; we'll just be able to release the good pages to the
>> page allocator and sequester the hwpoison pages.
>
> [+Cc PAGE ALLOCATOR folks]
>
> So we need an interface to free only healthy portion of a hwpoison folio.
>
> I think a proper approach to this should be to "free a hwpoison folio
> just like freeing a normal folio via folio_put() or free_frozen_pages(),
> then the page allocator will add only healthy pages to the freelist and
> isolate the hwpoison pages". Oherwise we'll end up open coding a lot,
> which is too fragile.

Why not use __split_unmaped_folio(folio, /*new_order=*/0,
								  /split_at=*/HWPoisoned_page,
								  ..., /*uniform_split=*/ false)?

If there are multiple HWPoisoned pages, just repeat.

>
> In fact, that can be done by teaching free_pages_prepare() how to handle
> the case where one or more subpages of a folio are hwpoison pages.
>
> How this should be implemented in the page allocator in memdescs world?
> Hmm, we'll want to do some kind of non-uniform split, without actually
> splitting the folio but allocating struct buddy?
>
> But... for now I think hiding this complexity inside the page allocator
> is good enough. For now this would just mean splitting a frozen page
> inside the page allocator (probably non-uniform?). We can later re-implement
> this to provide better support for memdescs.
>
> -- 
> Cheers,
> Harry / Hyeonggon


--
Best Regards,
Yan, Zi