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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: On Fri, Dec 12, 2025 at 08:10:17AM +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 page tables, these comments are outdated.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") > Reviewed-by: Rik van Riel > Tested-by: Laurence Oberman > Reviewed-by: Lorenzo Stoakes > Acked-by: Oscar Salvador > Cc: Liu Shixin > Signed-off-by: David Hildenbrand (Red Hat) > --- Looks good to me, Reviewed-by: Harry Yoo with a question below. > 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. > - * Instead, do flush now. Does this mean we can now try defer-freeing of these page tables, and if so, would it be worth it? > + * 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 -- Cheers, Harry / Hyeonggon