The patch titled Subject: mm/khugepaged: check again on anon uffd-wp during isolation has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-khugepaged-check-again-on-anon-uffd-wp-during-isolation.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Peter Xu peterx@redhat.com Subject: mm/khugepaged: check again on anon uffd-wp during isolation Date: Wed, 5 Apr 2023 11:51:20 -0400

Khugepaged collapse an anonymous thp in two rounds of scans. The 2nd round done in __collapse_huge_page_isolate() after hpage_collapse_scan_pmd(), during which all the locks will be released temporarily. It means the pgtable can change during this phase before 2nd round starts.

It's logically possible some ptes got wr-protected during this phase, and we can errornously collapse a thp without noticing some ptes are wr-protected by userfault. e1e267c7928f wanted to avoid it but it only did that for the 1st phase, not the 2nd phase.

Since __collapse_huge_page_isolate() happens after a round of small page swapins, we don't need to worry on any !present ptes - if it existed khugepaged will already bail out. So we only need to check present ptes with uffd-wp bit set there.

This is something I found only but never had a reproducer, I thought it was one caused a bug in Muhammad's recent pagemap new ioctl work, but it turns out it's not the cause of that but an userspace bug. However this seems to still be a real bug even with a very small race window, still worth to have it fixed and copy stable.

Link: https://lkml.kernel.org/r/20230405155120.3608140-1-peterx@redhat.com Fixes: e1e267c7928f ("khugepaged: skip collapse if uffd-wp detected") Signed-off-by: Peter Xu peterx@redhat.com Reviewed-by: David Hildenbrand david@redhat.com Reviewed-by: Yang Shi shy828301@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Axel Rasmussen axelrasmussen@google.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/khugepaged.c | 4 ++++ 1 file changed, 4 insertions(+)

--- a/mm/khugepaged.c~mm-khugepaged-check-again-on-anon-uffd-wp-during-isolation +++ a/mm/khugepaged.c @@ -573,6 +573,10 @@ static int __collapse_huge_page_isolate( result = SCAN_PTE_NON_PRESENT; goto out; } + if (pte_uffd_wp(pteval)) { + result = SCAN_PTE_UFFD_WP; + goto out; + } page = vm_normal_page(vma, address, pteval); if (unlikely(!page) || unlikely(is_zone_device_page(page))) { result = SCAN_PAGE_NULL; _

Patches currently in -mm which might be from peterx@redhat.com are

mm-hugetlb-fix-uffd-wr-protection-for-cow-optimization-path.patch mm-hugetlb-fix-uffd-wr-protection-for-cow-optimization-path-v2.patch mm-hugetlb-fix-uffd-wr-protection-for-cow-optimization-path-v3.patch mm-khugepaged-check-again-on-anon-uffd-wp-during-isolation.patch mm-uffd-uffd_feature_wp_unpopulated.patch mm-uffd-uffd_feature_wp_unpopulated-fix.patch selftests-mm-smoke-test-uffd_feature_wp_unpopulated.patch