path: root/lib
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authorMina Almasry <>2021-06-04 20:01:36 -0700
committerLinus Torvalds <>2021-06-05 08:58:12 -0700
commitd84cf06e3dd8c5c5b547b5d8931015fc536678e5 (patch)
tree4fb90c365b9f8e8c0ea8acc66fd83016df3fad52 /lib
parent7b6889f54a3c8c4139137a24a3ca12fe52a91dba (diff)
mm, hugetlb: fix simple resv_huge_pages underflow on UFFDIO_COPY
The userfaultfd hugetlb tests cause a resv_huge_pages underflow. This happens when hugetlb_mcopy_atomic_pte() is called with !is_continue on an index for which we already have a page in the cache. When this happens, we allocate a second page, double consuming the reservation, and then fail to insert the page into the cache and return -EEXIST. To fix this, we first check if there is a page in the cache which already consumed the reservation, and return -EEXIST immediately if so. There is still a rare condition where we fail to copy the page contents AND race with a call for hugetlb_no_page() for this index and again we will underflow resv_huge_pages. That is fixed in a more complicated patch not targeted for -stable. Test: Hacked the code locally such that resv_huge_pages underflows produce a warning, then: ./tools/testing/selftests/vm/userfaultfd hugetlb_shared 10 2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success ./tools/testing/selftests/vm/userfaultfd hugetlb 10 2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success Both tests succeed and produce no warnings. After the test runs number of free/resv hugepages is correct. [ changelog fixes] Link: Fixes: 8fb5debc5fcd ("userfaultfd: hugetlbfs: add hugetlb_mcopy_atomic_pte for userfaultfd support") Signed-off-by: Mina Almasry <> Reviewed-by: Mike Kravetz <> Cc: Axel Rasmussen <> Cc: Peter Xu <> Cc: <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
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