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    SubjectRe: [PATCH v3 03/14] mm, hugetlb: protect region tracking via newly introduced resv_map lock
    On Tue, Dec 24, 2013 at 11:00:12PM +1100, David Gibson wrote:
    > On Mon, Dec 23, 2013 at 10:05:17AM +0900, Joonsoo Kim wrote:
    > > On Sun, Dec 22, 2013 at 12:58:19AM +1100, David Gibson wrote:
    > > > On Wed, Dec 18, 2013 at 03:53:49PM +0900, Joonsoo Kim wrote:
    > > > > There is a race condition if we map a same file on different processes.
    > > > > Region tracking is protected by mmap_sem and hugetlb_instantiation_mutex.
    > > > > When we do mmap, we don't grab a hugetlb_instantiation_mutex, but,
    > > > > grab a mmap_sem. This doesn't prevent other process to modify region
    > > > > structure, so it can be modified by two processes concurrently.
    > > > >
    > > > > To solve this, I introduce a lock to resv_map and make region manipulation
    > > > > function grab a lock before they do actual work. This makes region
    > > > > tracking safe.
    > > >
    > > > It's not clear to me if you're saying there is a list corruption race
    > > > bug in the existing code, or only that there will be if the
    > > > instantiation mutex goes away.
    > >
    > > Hello,
    > >
    > > The race exists in current code.
    > > Currently, region tracking is protected by either down_write(&mm->mmap_sem) or
    > > down_read(&mm->mmap_sem) + instantiation mutex. But if we map this hugetlbfs
    > > file to two different processes, holding a mmap_sem doesn't have any impact on
    > > the other process and concurrent access to data structure is possible.
    >
    > Ouch. In that case:
    >
    > Acked-by: David Gibson <david@gibson.dropbear.id.au>
    >
    > It would be really nice to add a testcase for this race to the
    > libhugetlbfs testsuite.

    Okay!
    I will add it.

    Thanks.


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