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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Sep 18, 2025 at 11:30:48PM -0700, Lokesh Gidra wrote: > On Thu, Sep 18, 2025 at 5:38 AM Lorenzo Stoakes > wrote: > > > > On Wed, Sep 17, 2025 at 10:51:35PM -0700, Lokesh Gidra wrote: > > > Now that rmap_walk() is guaranteed to be called with the folio lock > > > held, we can stop serializing on the src VMA anon_vma lock when moving > > > an exclusive folio from a src VMA to a dst VMA in UFFDIO_MOVE ioctl. > > > > > > When moving a folio, we modify folio->mapping through > > > folio_move_anon_rmap() and adjust folio->index accordingly. Doing that > > > while we could have concurrent RMAP walks would be dangerous. Therefore, > > > to avoid that, we had to acquire anon_vma of src VMA in write-mode. That > > > meant that when multiple threads called UFFDIO_MOVE concurrently on > > > distinct pages of the same src VMA, they would serialize on it, hurting > > > scalability. > > > > > > In addition to avoiding the scalability bottleneck, this patch also > > > simplifies the complicated lock dance that UFFDIO_MOVE has to go through > > > between RCU, folio-lock, ptl, and anon_vma. > > > > > > folio_move_anon_rmap() already enforces that the folio is locked. So > > > when we have the folio locked we can no longer race with concurrent > > > rmap_walk() as used by folio_referenced() and hence the anon_vma lock > > > > And other rmap callers right? > Right. Will fix it in the next version. Thanks! > > > > > is no longer required. > > > > > > Note that this handling is now the same as for other > > > folio_move_anon_rmap() users that also do not hold the anon_vma lock -- > > > namely COW reuse handling. These users never required the anon_vma lock > > > as they are only moving the anon VMA closer to the anon_vma leaf of the > > > VMA, for example, from an anon_vma root to a leaf of that root. rmap > > > walks were always able to tolerate that scenario. > > > > Which users? > > The COW reusers, namely: > do_wp_page()->wp_can_reuse_anon_folio() > do_huge_pmd_wp_page() > hugetlb_wp() Right let's put this in the commit message is what I mean :) > > > > > > > > > CC: David Hildenbrand > > > CC: Lorenzo Stoakes > > > CC: Peter Xu > > > CC: Suren Baghdasaryan > > > CC: Barry Song > > > Signed-off-by: Lokesh Gidra > > > --- > > > mm/huge_memory.c | 22 +---------------- > > > mm/userfaultfd.c | 62 +++++++++--------------------------------------- > > > 2 files changed, 12 insertions(+), 72 deletions(-) > > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > index 5acca24bbabb..f444c142a8be 100644 > > > --- a/mm/huge_memory.c > > > +++ b/mm/huge_memory.c > > > @@ -2533,7 +2533,6 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm > > > pmd_t _dst_pmd, src_pmdval; > > > struct page *src_page; > > > struct folio *src_folio; > > > - struct anon_vma *src_anon_vma; > > > spinlock_t *src_ptl, *dst_ptl; > > > pgtable_t src_pgtable; > > > struct mmu_notifier_range range; > > > @@ -2582,23 +2581,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm > > > src_addr + HPAGE_PMD_SIZE); > > > mmu_notifier_invalidate_range_start(&range); > > > > > > - if (src_folio) { > > > + if (src_folio) > > > folio_lock(src_folio); > > > > > > - /* > > > - * split_huge_page walks the anon_vma chain without the page > > > - * lock. Serialize against it with the anon_vma lock, the page > > > - * lock is not enough. > > > - */ > > > - src_anon_vma = folio_get_anon_vma(src_folio); > > > - if (!src_anon_vma) { > > > - err = -EAGAIN; > > > - goto unlock_folio; > > > - } > > > - anon_vma_lock_write(src_anon_vma); > > > - } else > > > - src_anon_vma = NULL; > > > - > > > > Hmm this seems an odd thing to include in the uffd change. Why not just include > > it in the last commit or as a separate commit? You're changing move_pages_huge_pmd() here in a change that's about the uffd change, seems unrelated no? > > I'm not sure I follow. What am I including here? > > BTW, IMHO, the comment is wrong here. folio split code already > acquires folio lock. The anon_vma lock is required here for the same > reason as non-large page case - to avoid concurrent rmap walks. This is called via split_huge_page() used by KMS and memory failure, not the usual folio split logic afaict. But those callers all take the folio look afaict :) So yeah the comment is wrong it seems!