linux-kernel - Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages

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: <b10e3b2a-b298-4d27-b8ce-63327864c220@kernel.org>
Date: Thu, 15 Jan 2026 18:41:44 +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>,
 Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com>,
 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>,
 kernel-team@...a.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages

On 1/15/26 18:23, Kiryl Shutsemau wrote:
> On Thu, Jan 15, 2026 at 05:49:43PM +0100, David Hildenbrand (Red Hat) wrote:
>> On 1/15/26 15:45, Kiryl Shutsemau wrote:
>>> HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most
>>> vmemmap pages for huge pages and remapping the freed range to a single
>>> page containing the struct page metadata.
>>>
>>> With the new mask-based compound_info encoding (for power-of-2 struct
>>> page sizes), all tail pages of the same order are now identical
>>> regardless of which compound page they belong to. This means the tail
>>> pages can be truly shared without fake heads.
>>>
>>> Allocate a single page of initialized tail struct pages per NUMA node
>>> per order in the vmemmap_tails[] array in pglist_data. All huge pages
>>> of that order on the node share this tail page, mapped read-only into
>>> their vmemmap. The head page remains unique per huge page.
>>>
>>> This eliminates fake heads while maintaining the same memory savings,
>>> and simplifies compound_head() by removing fake head detection.
>>>
>>> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
>>> ---
>>>    include/linux/mmzone.h | 16 ++++++++++++++-
>>>    mm/hugetlb_vmemmap.c   | 44 ++++++++++++++++++++++++++++++++++++++++--
>>>    mm/sparse-vmemmap.c    | 44 ++++++++++++++++++++++++++++++++++--------
>>>    3 files changed, 93 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index 322ed4c42cfc..2ee3eb610291 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -82,7 +82,11 @@
>>>     * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
>>>     * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
>>>     */
>>> -#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
>>> +#ifdef CONFIG_64BIT
>>> +#define MAX_FOLIO_ORDER		(34 - PAGE_SHIFT)
>>> +#else
>>> +#define MAX_FOLIO_ORDER		(30 - PAGE_SHIFT)
>>> +#endif
>>
>> Where do these magic values stem from, and how do they related to the
>> comment above that clearly spells out 16G vs. 1G ?
> 
> This doesn't change the resulting value: 1UL << 34 is 16GiB, 1UL << 30
> is 1G. Subtract PAGE_SHIFT to get the order.
> 
> The change allows the value to be used to define NR_VMEMMAP_TAILS which
> is used specify size of vmemmap_tails array.

get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) should evaluate to 
a constant by the compiler.

See __builtin_constant_p handling in get_order().

If that is not working then we have to figure out why.

Was this only a specific config in where you ran into compile-time problems?

-- 
Cheers

David