From: Peter Xu Date: Wed, 5 Apr 2023 15:51:20 +0000 (-0400) Subject: mm/khugepaged: check again on anon uffd-wp during isolation X-Git-Tag: v6.1.37~2076 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=519dbe737f0d26b2c34a1f3990bf27dd82d3778d;p=platform%2Fkernel%2Flinux-starfive.git mm/khugepaged: check again on anon uffd-wp during isolation commit dd47ac428c3f5f3bcabe845f36be870fe6c20784 upstream. 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 Reviewed-by: David Hildenbrand Reviewed-by: Yang Shi Cc: Andrea Arcangeli Cc: Axel Rasmussen Cc: Mike Rapoport Cc: Nadav Amit Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 77a76bc..ef72d3d 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -561,6 +561,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, 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;