The patch titled Subject: mm/hugetlb: fix hugetlb_pmd_shared() has been added to the -mm mm-new branch. Its filename is mm-hugetlb-fix-hugetlb_pmd_shared.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

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------------------------------------------------------ From: "David Hildenbrand (Red Hat)" david@kernel.org Subject: mm/hugetlb: fix hugetlb_pmd_shared() Date: Fri, 12 Dec 2025 08:10:16 +0100

Patch series "mm/hugetlb: fixes for PMD table sharing (incl. using mmu_gather)", v2.

One functional fix, one performance regression fix, and two related comment fixes.

The goal of this patch set is to be backported to stable trees "fairly" easily. At least patch #1 and #4.

Patch #1 fixes hugetlb_pmd_shared() not detecting any sharing Patch #2 + #3 are simple comment fixes that patch #4 interacts with. Patch #4 is a fix for the reported performance regression due to excessive IPI broadcasts during fork()+exit().

The last patch is all about TLB flushes, IPIs and mmu_gather. Read: complicated

This patch (of 4):

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().

Link: https://lkml.kernel.org/r/20251212071019.471146-1-david@kernel.org Link: https://lkml.kernel.org/r/20251212071019.471146-2-david@kernel.org Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: David Hildenbrand (Red Hat) david@kernel.org Reviewed-by: Rik van Riel riel@surriel.com Reviewed-by: Lance Yang lance.yang@linux.dev Tested-by: Lance Yang lance.yang@linux.dev Reviewed-by: Harry Yoo harry.yoo@oracle.com Tested-by: Laurence Oberman loberman@redhat.com Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com Acked-by: Oscar Salvador osalvador@suse.de Cc: Liu Shixin liushixin2@huawei.com Cc: "Aneesh Kumar K.V" aneesh.kumar@kernel.org Cc: Arnd Bergmann arnd@arndb.de Cc: Jann Horn jannh@google.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Muchun Song muchun.song@linux.dev Cc: Nadav Amit nadav.amit@gmail.com Cc: Nicholas Piggin npiggin@gmail.com Cc: Peter Zijlstra peterz@infradead.org Cc: Prakash Sangappa prakash.sangappa@oracle.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Will Deacon will@kernel.org Cc: Uschakow, Stanislav" suschako@amazon.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/hugetlb.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/hugetlb.h~mm-hugetlb-fix-hugetlb_pmd_shared +++ a/include/linux/hugetlb.h @@ -1326,7 +1326,7 @@ static inline __init void hugetlb_cma_re #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING static inline bool hugetlb_pmd_shared(pte_t *pte) { - return page_count(virt_to_page(pte)) > 1; + return ptdesc_pmd_is_shared(virt_to_ptdesc(pte)); } #else static inline bool hugetlb_pmd_shared(pte_t *pte) _

Patches currently in -mm which might be from david@kernel.org are

mm-hugetlb-fix-hugetlb_pmd_shared.patch mm-hugetlb-fix-two-comments-related-to-huge_pmd_unshare.patch mm-rmap-fix-two-comments-related-to-huge_pmd_unshare.patch mm-hugetlb-fix-excessive-ipi-broadcasts-when-unsharing-pmd-tables-using-mmu_gather.patch