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    SubjectRe: [rfc][patch] x86: avoid highmem cache attribute aliasing
    On Mon, Aug 11, 2008 at 10:10:19AM -0700, Ingo Molnar wrote:
    >
    > * Nick Piggin <npiggin@suse.de> wrote:
    >
    > > Highmem code can leave ptes and tlb entries around for a given page
    > > even after kunmap, and after it has been freed.
    > >
    > > From what I can gather, the PAT code may change the cache attributes
    > > of arbitrary physical addresses (ie. including highmem pages), which
    > > would result in aliases in the case that it operates on one of these
    > > lazy tlb highmem pages.
    > >
    > > Flushing kmaps should solve the problem.
    > >
    > > I've also just added code for conditional flushing if we haven't got
    > > any dangling highmem aliases -- this should help performance if we
    > > change page attributes frequently or systems that aren't using much
    > > highmem pages (eg. if < 4G RAM). Should be turned into 2 patches, but
    > > just for RFC...
    >
    > hm, such aliasing might happen in theory - and i guess in practice too
    > if the AGP driver allocates/deallocates aperture in short succession.
    >
    > Maybe this could corrupt the X framebuffer - or even generic kernel RAM.
    > Mind resending the two split up patches? The fix we might want to take
    > into v2.6.27, the speedup probably for v2.6.28.
    >
    > But maybe i'm missing something obvious that prevents such problems on
    > 32-bit systems - Venki, Suresh, what do you think?

    Andi pointed out in another thread that we don't use GFP_HIGHMEM in the
    AGP drivers. And hence we haven't encountered an issue so far. But yes,
    this is a good fix to close the hole.

    thanks,
    suresh


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