Re: [PATCH v4] mm, hugetlb: Fix simple resv_huge_pages underflow on UFFDIO_COPY

[Mike Kravetz <mike.kravetz@oracle.com>]

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 <almasrymina@google.com>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> 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 <mike.kravetz@oracle.com>

-- 
Mike Kravetz