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/mm-khugepaged-check-again-on-anon-uffd-wp-during-isolation.patch
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@xxxxxxxxxx>
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@xxxxxxxxxx
Fixes: e1e267c7928f ("khugepaged: skip collapse if uffd-wp detected")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxx 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
