[PATCH 4.19 39/58] mm, hugetlb: fix simple resv_huge_pages underflow on UFFDIO_COPY

Greg Kroah-Hartman
2021-06-08T18:43:58Z

From: Mina Almasry <almasrymina@google.com>

[ Upstream commit d84cf06e3dd8c5c5b547b5d8931015fc536678e5 ]

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.

[mike.kravetz@oracle.com: changelog fixes]

Link: https://lkml.kernel.org/r/20210528004649.85298-1-almasrymina@google.com
Fixes: 8fb5debc5fcd ("userfaultfd: hugetlbfs: add hugetlb_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/hugetlb.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2f769a661568..c69f12e4c149 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4154,10 +4154,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
struct page *page;

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,
--
2.30.2