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

[Andrew Morton]

On Thu, 27 May 2021 17:46:49 -0700 Mina Almasry <almasrymina@xxxxxxxxxx> wrote:

> The userfaultfd hugetlb tests detect 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
> 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@xxxxxxxxxx>
> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx

Do we have a Fixes: for this, or is it an always-been-there issue?