linux-kernel - Re: [PATCH v1 1/4] mm/hugetlb: fix hugetlb_pmd

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Message-ID: <246da543-58a9-4b61-8c88-5ceaca9dc12a@kernel.org>
Date: Mon, 8 Dec 2025 12:01:49 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lance Yang <ioworker0@...il.com>
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org,
 aneesh.kumar@...nel.org, arnd@...db.de, harry.yoo@...cle.com,
 jannh@...gle.com, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, liushixin2@...wei.com, loberman@...hat.com,
 lorenzo.stoakes@...cle.com, muchun.song@...ux.dev, nadav.amit@...il.com,
 npiggin@...il.com, osalvador@...e.de, peterz@...radead.org,
 pfalcato@...e.de, prakash.sangappa@...cle.com, riel@...riel.com,
 stable@...r.kernel.org, vbabka@...e.cz, will@...nel.org,
 Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v1 1/4] mm/hugetlb: fix hugetlb_pmd_shared()

On 12/8/25 03:32, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> 
> On Fri,  5 Dec 2025 22:35:55 +0100, David Hildenbrand (Red Hat) wrote:
>> We switched from (wrongly) using the page count to an independent
>> shared count. Now, shared page tables have a refcount of 1 (excluding
>> speculative references) and instead use ptdesc->pt_share_count to
>> identify sharing.
>>
>> We didn't convert hugetlb_pmd_shared(), so right now, we would never
>> detect a shared PMD table as such, because sharing/unsharing no longer
>> touches the refcount of a PMD table.
>>
>> Page migration, like mbind() or migrate_pages() would allow for migrating
>> folios mapped into such shared PMD tables, even though the folios are
>> not exclusive. In smaps we would account them as "private" although they
>> are "shared", and we would be wrongly setting the PM_MMAP_EXCLUSIVE in the
>> pagemap interface.
>>
>> Fix it by properly using ptdesc_pmd_is_shared() in hugetlb_pmd_shared().
>>
>> Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count")
>> Cc: <stable@...r.kernel.org>
>> Cc: Liu Shixin <liushixin2@...wei.com>
>> Signed-off-by: David Hildenbrand (Red Hat) <david@...nel.org>
>> ---
> 
> Tested on x86 with two independent processes sharing a 1GiB hugetlbfs file
> (aligned a 1GiB boundary).
> 
> Before the fix, even though PMD sharing worked (pt_share_count=1),
> hugetlb_pmd_shared() returned false because page_count() was still 1,
> causing smaps to report it as "Private" and pagemap to set it
> PM_MMAP_EXCLUSIVE incorrectly :(
> 
> After the fix, hugetlb_pmd_shared() correctly detects the sharing, smaps
> reports it as "Shared", and PM_MMAP_EXCLUSIVE is cleared ;)
> 
> Tested-by: Lance Yang <lance.yang@...ux.dev>

Thanks a lot Lance for the testing and thanks to everybody for the review!

-- 
Cheers

David