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

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Message-ID: <aTaVkTQFqX96UinR@hyeyoo>
Date: Mon, 8 Dec 2025 18:08:33 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, Will Deacon <will@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>, Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
        Laurence Oberman <loberman@...hat.com>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>, stable@...r.kernel.org,
        Liu Shixin <liushixin2@...wei.com>
Subject: Re: [PATCH v1 1/4] mm/hugetlb: fix hugetlb_pmd_shared()

On Fri, Dec 05, 2025 at 10:35:55PM +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>
> ---

Oops, didn't notice there's still missing conversion!

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

-- 
Cheers,
Harry / Hyeonggon