linux-kernel - Re: [PATCH v4] mm, hugetlb: Fix simple resv_huge_pages underflow on UFFDIO

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Date:   Fri, 28 May 2021 11:43:48 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v4] mm, hugetlb: Fix simple resv_huge_pages underflow on
 UFFDIO_COPY

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@...gle.com>
> Cc: Axel Rasmussen <axelrasmussen@...gle.com>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: linux-mm@...ck.org
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> Cc: stable@...r.kernel.org
> 
> ---
>  mm/hugetlb.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)

Code changes are fine.  Thanks,

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz