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    SubjectRe: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments


    On Mon, 2 Feb 2009, Mel Gorman wrote:
    >
    > Do not account for address space usage when making hugetlbfs mappings RW
    >
    > hugetlbfs accounts for its address space usage separate from the VM
    > core. VM_ACCOUNT should not be set for its mappings but it is possible it gets
    > set if a user creates a RO hugetlbfs mapping MAP_NORESERVE and then calls
    > mprotect(). This patch stops VM_ACCOUNT being set for hugetlbfs mappings
    > during mprotect().
    >
    > Credit goes to Kosaki Motohiro for spotting this.
    >
    > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    >
    > diff --git a/mm/mprotect.c b/mm/mprotect.c
    > index abe2694..31ddc6a 100644
    > --- a/mm/mprotect.c
    > +++ b/mm/mprotect.c
    > @@ -153,7 +153,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
    > * but (without finer accounting) cannot reduce our commit if we
    > * make it unwritable again.
    > */
    > - if (newflags & VM_WRITE) {
    > + if (newflags & VM_WRITE && !(vma->vm_flags & VM_HUGETLB)) {

    Wouldn't it be _much_ nicer to just depend on that whole VM_NORESERVE
    thing?

    Those hugetlb mappings _should_ have VM_NORESERVE on them, so the
    following test:

    > if (!(oldflags & (VM_ACCOUNT|VM_WRITE|
    > VM_SHARED|VM_NORESERVE))) {
    > charged = nrpages;

    should do it all correctly.

    Why make up some ad-hoc testing, when we already have a flag for _exactly_
    this issue. That's what VM_NORESERVE means: don't apply VM_ACCOUNT.

    IOW, I don't see the point of this patch at all.

    And if there is some hugetlb path that doesn't set VM_NORESERVE, then fix
    _that_ instead.

    Linus


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