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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Rik van Riel , Harry Yoo , Laurence Oberman , Prakash Sangappa , Nadav Amit , Liu Shixin Subject: Re: [PATCH v1 2/4] mm/hugetlb: fix two comments related to huge_pmd_unshare() Message-ID: <834ec5ca-d43c-441d-a10b-ea268333e433@lucifer.local> References: <20251205213558.2980480-1-david@kernel.org> <20251205213558.2980480-3-david@kernel.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251205213558.2980480-3-david@kernel.org> X-ClientProxiedBy: LO4P302CA0044.GBRP302.PROD.OUTLOOK.COM (2603:10a6:600:317::10) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|CH2PR10MB4328:EE_ X-MS-Office365-Filtering-Correlation-Id: 891da33f-5923-4f5e-91d3-08de37de6ed3 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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List-Subscribe: List-Unsubscribe: On Fri, Dec 05, 2025 at 10:35:56PM +0100, David Hildenbrand (Red Hat) wrote: > Ever since we stopped using the page count to detect shared PMD > page tables, these comments are outdated. > > The only reason we have to flush the TLB early is because once we drop > the i_mmap_rwsem, the previously shared page table could get freed (to > then get reallocated and used for other purpose). So we really have to > flush the TLB before that could happen. > > So let's simplify the comments a bit. > > The "If we unshared PMDs, the TLB flush was not recorded in mmu_gather." > part introduced as in commit a4a118f2eead ("hugetlbfs: flush TLBs > correctly after huge_pmd_unshare") was confusing: sure it is recorded > in the mmu_gather, otherwise tlb_flush_mmu_tlbonly() wouldn't do > anything. So let's drop that comment while at it as well. > > We'll centralize these comments in a single helper as we rework the code > next. > > Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") > Cc: Liu Shixin > Signed-off-by: David Hildenbrand (Red Hat) LGTM, so: Reviewed-by: Lorenzo Stoakes > --- > mm/hugetlb.c | 24 ++++++++---------------- > 1 file changed, 8 insertions(+), 16 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 51273baec9e5d..3c77cdef12a32 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -5304,17 +5304,10 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma, > tlb_end_vma(tlb, vma); > > /* > - * If we unshared PMDs, the TLB flush was not recorded in mmu_gather. We > - * could defer the flush until now, since by holding i_mmap_rwsem we > - * guaranteed that the last reference would not be dropped. But we must > - * do the flushing before we return, as otherwise i_mmap_rwsem will be > - * dropped and the last reference to the shared PMDs page might be > - * dropped as well. > - * > - * In theory we could defer the freeing of the PMD pages as well, but > - * huge_pmd_unshare() relies on the exact page_count for the PMD page to > - * detect sharing, so we cannot defer the release of the page either. Was it this comment that led you to question the page_count issue? :) > - * Instead, do flush now. > + * There is nothing protecting a previously-shared page table that we > + * unshared through huge_pmd_unshare() from getting freed after we > + * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare() > + * succeeded, flush the range corresponding to the pud. > */ > if (force_flush) > tlb_flush_mmu_tlbonly(tlb); > @@ -6536,11 +6529,10 @@ long hugetlb_change_protection(struct vm_area_struct *vma, > cond_resched(); > } > /* > - * Must flush TLB before releasing i_mmap_rwsem: x86's huge_pmd_unshare > - * may have cleared our pud entry and done put_page on the page table: > - * once we release i_mmap_rwsem, another task can do the final put_page > - * and that page table be reused and filled with junk. If we actually > - * did unshare a page of pmds, flush the range corresponding to the pud. > + * There is nothing protecting a previously-shared page table that we > + * unshared through huge_pmd_unshare() from getting freed after we > + * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare() > + * succeeded, flush the range corresponding to the pud. > */ > if (shared_pmd) > flush_hugetlb_tlb_range(vma, range.start, range.end); > -- > 2.52.0 >