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

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Message-ID: <CAHS8izO_-wK8GL9iK6HiG5xJc1unMLxO0uzFF20v4HKU36R8Ug@mail.gmail.com>
Date:   Mon, 31 May 2021 19:49:01 -0700
From:   Mina Almasry <almasrymina@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        open list <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 Mon, May 31, 2021 at 5:37 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Thu, 27 May 2021 17:46:49 -0700 Mina Almasry <almasrymina@...gle.com> 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@...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
>
> Do we have a Fixes: for this, or is it an always-been-there issue?

This reproduces as far back as 4.15 kernel. Looks like always-been-there issue.