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    SubjectRe: 2.6.29 pat issue
    Eric W. Biederman wrote:
    > Thomas Hellstrom <> writes:
    >> Indeed, it's crucial to keep the mappings consistent, but failure to do so is a
    >> kernel driver bug, it should never be the result of invalid user data.
    > It easily can be. Think of an X server mmaping frame buffers. Or other
    > device bars.
    Hmm, Yes you're right, although I'm still a bit doubtful about RAM pages.

    Wait. Now I see what's causing the problems. The code is assuming that
    VM_PFNMAP vmas never map RAM pages. That's also an invalid assumption.
    See comments in mm/memory.c

    So probably the attribute check should be done for the insert_pfn path
    of VM_MIXEDMAP as well. That's not done today.

    So there are three distinct bugs at this point:

    1) VMAs with VM_PFNMAP are incorrectly assumed to be linear if
    vma->vm_pgoff non-null.
    2) VM_PFNMAP VMA PTEs are incorrectly assumed to never point to physical
    3) There is no check for the insert_pfn path of vm_insert_mixed().

    >> IMHO checking each vm_insert_pfn() for caching attribute correctness is not
    >> something that should be enabled by default, due to the CPU overhead. Production
    >> drivers should never violate this.
    > If it is a problem the implementation should become more efficient. Userspace
    > as well as drivers can generate these mappings so even with a perfect driver
    > you cannot guarantee that someone else does not have that area of memory
    > mapped differently.
    OK, So there seems to be a couple of things that can be done for
    performance here:

    1) A fastpath for single pages.
    2) RAM pages are tracked with a page bit today.
    Why not say "all memory backed by a struct page" should be tracked with
    a page bit. Then pfn_valid() could be used instead of page_is_ram().
    This, combined with 1) should make tracking struct page backed pages
    extremely fast.
    3) If vm_insert_pfn() happens to be used on a linear VMA, it looks like
    the whole VMA is being validated for each vm_insert_pfn(), which seems
    extremely inefficient, considering the extensive tests in pagerame_is_ram().


    > Eric

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      Last update: 2009-02-06 10:47    [W:0.030 / U:136.260 seconds]
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