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b=QgSw5nHKaxkwnoKwCn/aVrI+qzJmMvWbxLRmvY2sEDoM5fT6vxafkh4zx1sStkn/B QWBeHqdSg496oR5Q6bblxNmqgzKEgAjcLLJDegjFibcBoJH/XA8O8KmJg9cr3DnEPl +ru+nu6MF569JAQXdpwQOon/Is/Bq9ulwmBpkAp+aYHCTEVYiJX8dXeseWKV1vrJJB /cJy6gi2tjCtL2A0s+mGbh125h7qLIHt7r1jxpqltJYdLLYeE94gmQSE/e03Wq3Qlg Ng9ZerRxJuquBoOlLFwhpDGwD8xX3ieZsIXwkmyr+R0pi1bQze65053FTM0d3qy6ju MiF14gGpOC9rg== From: "David Hildenbrand (Arm)" To: stable@vger.kernel.org Cc: linux-mm@kvack.org, "David Hildenbrand (Red Hat)" , Rik van Riel , Lance Yang , Harry Yoo , Laurence Oberman , Lorenzo Stoakes , Oscar Salvador , Liu Shixin , Andrew Morton Subject: [PATCH 5.15.y 3/6] mm/hugetlb: fix hugetlb_pmd_shared() Date: Wed, 18 Feb 2026 12:01:26 +0100 Message-ID: <20260218110129.41578-4-david@kernel.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260218110129.41578-1-david@kernel.org> References: <2026012608-tulip-moisten-c6f6@gregkh> <20260218110129.41578-1-david@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: uotznb5ynuj5bw88meskry7sxmhtxyww X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: CD49140009 X-HE-Tag: 1771412506-653960 X-HE-Meta: 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 rc/VXEAr Kwz4FTYKupA2UMGiJy2tDcyKU69idxAee2Jlo9Wo4tdQaZde/Q+B1u2M37ZTOsYDXxL0udbiSXLfqKbWBfxKztB+P1mVI7B155AsNdoiUQAAolrNcluna6wvd8acYSaxmLUMR3jx3k5wNdXOP/V43wXHTul+pk5ZmipmHJIZMH+d2MsSmN9od5QpEXMyq9xQn56xUHtoGv6jIHBRfksGRkHr5H8l1D6a7NMM3T452urpZZKprLSQdrpUjzYpLkRvoWWibhtrrn8xQ7DIvKGv1BrhHNw9W+6rTN/ubmBK9hFSo+MsHnalZJq2pCW7E2z82OZRB37tXeBZwaBlNLCEROdQe84wII52I93Yai/rO3Y81exDyPwD5nH0KagzHzHgo+LtWhvoZhXOlea9Dy7SaPsqs3Gg42bBVRWEJ8eOGVn7+TVwPOOl1nnpkaSRBSllLIRrf4++WQnSOVV0RtL2ov7yBRg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: "David Hildenbrand (Red Hat)" Patch series "mm/hugetlb: fixes for PMD table sharing (incl. using mmu_gather)", v3. One functional fix, one performance regression fix, and two related comment fixes. I cleaned up my prototype I recently shared [1] for the performance fix, deferring most of the cleanups I had in the prototype to a later point. While doing that I identified the other things. 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 There are plenty of cleanups in the future to be had + one reasonable optimization on x86. But that's all out of scope for this series. Runtime tested, with a focus on fixing the performance regression using the original reproducer [2] on x86. 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/20251223214037.580860-1-david@kernel.org Link: https://lkml.kernel.org/r/20251223214037.580860-2-david@kernel.org Link: https://lore.kernel.org/all/8cab934d-4a56-44aa-b641-bfd7e23bd673@kernel.org/ [1] Link: https://lore.kernel.org/all/8cab934d-4a56-44aa-b641-bfd7e23bd673@kernel.org/ [2] Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: David Hildenbrand (Red Hat) Reviewed-by: Rik van Riel Reviewed-by: Lance Yang Tested-by: Lance Yang Reviewed-by: Harry Yoo Tested-by: Laurence Oberman Reviewed-by: Lorenzo Stoakes Acked-by: Oscar Salvador Cc: Liu Shixin Cc: Uschakow, Stanislav" Cc: Signed-off-by: Andrew Morton (cherry picked from commit ca1a47cd3f5f4c46ca188b1c9a27af87d1ab2216) [ David: We don't have ptdesc and the wrappers, so work directly on page->pt_share_count. ] Signed-off-by: David Hildenbrand (Arm) --- include/linux/hugetlb.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h index 60572d423586..76011d445adc 100644 --- a/include/linux/hugetlb.h +++ b/include/linux/hugetlb.h @@ -1110,7 +1110,7 @@ static inline __init void hugetlb_cma_check(void) #ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE static inline bool hugetlb_pmd_shared(pte_t *pte) { - return page_count(virt_to_page(pte)) > 1; + return atomic_read(&virt_to_page(pte)->pt_share_count); } #else static inline bool hugetlb_pmd_shared(pte_t *pte) -- 2.43.0