linux-kernel - Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap optimization

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: <01e5d0b3-dbf8-4f77-b38a-f48c46f7c31e@kernel.org>
Date: Fri, 5 Dec 2025 21:44:30 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Matthew Wilcox <willy@...radead.org>,
 Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>,
 Usama Arif <usamaarif642@...il.com>, kernel-team@...a.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap
 optimization

On 12/5/25 21:33, Kiryl Shutsemau wrote:
> On Fri, Dec 05, 2025 at 09:16:08PM +0100, David Hildenbrand (Red Hat) wrote:
>> On 12/5/25 20:43, Kiryl Shutsemau wrote:
>>> This series removes "fake head pages" from the HugeTLB vmemmap
>>> optimization (HVO) by changing how tail pages encode their relationship
>>> to the head page.
>>>
>>> It simplifies compound_head() and page_ref_add_unless(). Both are in the
>>> hot path.
>>>
>>> Background
>>> ==========
>>>
>>> HVO reduces memory overhead by freeing vmemmap pages for HugeTLB pages
>>> and remapping the freed virtual addresses to a single physical page.
>>> Previously, all tail page vmemmap entries were remapped to the first
>>> vmemmap page (containing the head struct page), creating "fake heads" -
>>> tail pages that appear to have PG_head set when accessed through the
>>> deduplicated vmemmap.
>>>
>>> This required special handling in compound_head() to detect and work
>>> around fake heads, adding complexity and overhead to a very hot path.
>>>
>>> New Approach
>>> ============
>>>
>>> For architectures/configs where sizeof(struct page) is a power of 2 (the
>>> common case), this series changes how position of the head page is encoded
>>> in the tail pages.
>>>
>>> Instead of storing a pointer to the head page, the ->compound_info
>>> (renamed from ->compound_head) now stores a mask.
>>
>> (we're in the merge window)
>>
>> That doesn't seem to be suitable for the memdesc plans, where we want all
>> tail pages do directly point at the allocated memdesc (e.g., struct folio),
>> no?
> 
> Sure. My understanding is that it is going to eliminate a need in
> compound_head() completely. I don't see the conflict so far.

Right. All compound_head pointers will point at the allocated memdesc.

Would we still have to detect fake head pages though (at least for some 
transition period)?

I don't recall whether we'll really convert all memdesc users at once, 
or if some memdescs will co-exist with ordinary compound pages for a while.

-- 
Cheers

David