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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 19, 2025 at 07:11:00AM +0100, David Hildenbrand (Red Hat) wrote: > On 12/19/25 05:44, Harry Yoo wrote: > > 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 page tables, these comments are outdated.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. > > Hi Harry, > > thanks for the review! No problem! I would love to review more, as long as my time & ability allows ;) > > > 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 one very tricky thing: > > Whoever is the last owner of a (previously) shared page table must unmap any > contained pages (adjust mapcount/ref, sync a/d bit, ...). Right. > So it's not just a matter of deferring the freeing, because these page tables > will still contain content. I was (and maybe still) bit confused while reading the old comment as it implied (or maybe I just misread) that by deferring freeing of page tables we don't have to flush TLB in __unmap_hugepage_range() and can flush later instead. > I first tried to never allow for reuse of shared page tables, but precisely > that resulted in most headakes. I see your pain there... > So I don't see an easy way to achieve that (and I'm also not sure if we want > to add any further complexity to this). Fair enough. Thanks for answering! > -- > Cheers > > David -- Cheers, Harry / Hyeonggon