On 5/27/21 5:46 PM, Mina Almasry wrote:

The userfaultfd hugetlb tests detect a resv_huge_pages underflow. This

Perhaps say,
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 if there exists a page in the cache which already

To fix this, we first check if there is a page in the cache which already

To fix this, we first if there exists 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.

Signed-off-by: Mina Almasry <redacted>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: linux-mm@kvack.org
Cc: Mike Kravetz <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org

---
 mm/hugetlb.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

Code changes are fine.  Thanks,

Reviewed-by: Mike Kravetz <redacted>

-- 
Mike Kravetz