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    SubjectRe: [PATCH 2/3] hugetlbfs: close race between MADV_DONTNEED and page fault
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    On Tue, 2023-10-03 at 13:19 -0700, Mike Kravetz wrote:
    > On 10/03/23 15:35, Rik van Riel wrote:
    > > On Sun, 2023-10-01 at 21:39 -0700, Mike Kravetz wrote:
    > > >
    > > > Something is not right here.  I have not looked closely at the
    > > > patch,
    > > > but running libhugetlbfs test suite hits this NULL deref in
    > > > misalign
    > > > (2M: 32).
    > >
    > > Hi Mike,
    > >
    > > fixing the null dereference was easy, but I continued running
    > > into a test case failure with linkhuge_rw. After tweaking the
    > > code in my patches quite a few times, I finally ran out of
    > > ideas and tried it on a tree without my patches.
    > >
    > > I still see the test failure on upstream
    > > 2cf0f7156238 ("Merge tag 'nfs-for-6.6-2' of git://git.linux-
    > > nfs.org/projects/anna/linux-nfs")
    > >
    > > This is with a modern glibc, and the __morecore assignments
    > > in libhugetlbfs/morecore.c commented out.
    > >
    > >
    > > HUGETLB_ELFMAP=R HUGETLB_SHARE=1 linkhuge_rw (2M: 32):  Pool state:
    > > (('hugepages-2048kB', (('free_hugepages', 1), ('resv_hugepages',
    > > 0),
    > > ('surplus_hugepages', 0), ('nr_hugepages_mempolicy', 1),
    > > ('nr_hugepages', 1), ('nr_overcommit_hugepages', 0))),)
    > > Hugepage pool state not preserved!
    > > BEFORE: (('hugepages-2048kB', (('free_hugepages', 1),
    > > ('resv_hugepages', 0), ('surplus_hugepages', 0),
    > > ('nr_hugepages_mempolicy', 1), ('nr_hugepages', 1),
    > > ('nr_overcommit_hugepages', 0))),)
    > > AFTER: (('hugepages-2048kB', (('free_hugepages', 0),
    > > ('resv_hugepages',
    > > 0), ('surplus_hugepages', 0), ('nr_hugepages_mempolicy', 1),
    > > ('nr_hugepages', 1), ('nr_overcommit_hugepages', 0))),)
    > >
    >
    > Please consider the above failures normal and expected.  That have
    > been
    > this way for many years.  Sorry for any waste of your time.
    >
    > Of course, if you would like to look into these you are welcome.

    I'm not too worried about the test cases returning failure,
    but having free_hugepages not go back to 1 after linkhuge_rw
    exits looks bad.

    In this case it appears that linkhuge_rw simply left behind
    a file in /dev/hugepages when it died, and removing that file
    returns free_hugepages back to what it should be.

    I guess I'll go run the test cases without -c 1 :)

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