• Mel Gorman's avatar
    Do not account for the address space used by hugetlbfs using VM_ACCOUNT · 5a6fe125
    Mel Gorman authored
    When overcommit is disabled, the core VM accounts for pages used by anonymous
    shared, private mappings and special mappings. It keeps track of VMAs that
    should be accounted for with VM_ACCOUNT and VMAs that never had a reserve
    with VM_NORESERVE.
    
    Overcommit for hugetlbfs is much riskier than overcommit for base pages
    due to contiguity requirements. It avoids overcommiting on both shared and
    private mappings using reservation counters that are checked and updated
    during mmap(). This ensures (within limits) that hugepages exist in the
    future when faults occurs or it is too easy to applications to be SIGKILLed.
    
    As hugetlbfs makes its own reservations of a different unit to the base page
    size, VM_ACCOUNT should never be set. Even if the units were correct, we would
    double account for the usage in the core VM and hugetlbfs. VM_NORESERVE may
    be set because an application can request no reserves be made for hugetlbfs
    at the risk of getting killed later.
    
    With commit fc8744ad, VM_NORESERVE and
    VM_ACCOUNT are getting unconditionally set for hugetlbfs-backed mappings. This
    breaks the accounting for both the core VM and hugetlbfs, can trigger an
    OOM storm when hugepage pools are too small lockups and corrupted counters
    otherwise are used. This patch brings hugetlbfs more in line with how the
    core VM treats VM_NORESERVE but prevents VM_ACCOUNT being set.
    Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    5a6fe125
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