diff options
authorMina Almasry <>2021-06-04 20:01:36 -0700
committerLinus Torvalds <>2021-06-05 08:58:12 -0700
commitd84cf06e3dd8c5c5b547b5d8931015fc536678e5 (patch)
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 <>
1 files changed, 12 insertions, 2 deletions
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 470f7b5b437e..5560b50876fb 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4889,10 +4889,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
if (!page)
goto out;
} else if (!*pagep) {
- ret = -ENOMEM;
+ /* If a page already exists, then it's UFFDIO_COPY for
+ * a non-missing case. Return -EEXIST.
+ */
+ if (vm_shared &&
+ hugetlbfs_pagecache_present(h, dst_vma, dst_addr)) {
+ ret = -EEXIST;
+ goto out;
+ }
page = alloc_huge_page(dst_vma, dst_addr, 0);
- if (IS_ERR(page))
+ if (IS_ERR(page)) {
+ ret = -ENOMEM;
goto out;
+ }
ret = copy_huge_page_from_user(page,
(const void __user *) src_addr,