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    SubjectRe: [patch 1/7] x86 PAT: store vm_pgoff for all linear_over_vma_region mappings - v3
    On Thu, Dec 18, 2008 at 02:10:57PM -0800, Pallipadi, Venkatesh wrote:
    > On Thu, Dec 18, 2008 at 01:27:28PM -0800, Nick Piggin wrote:
    > >
    > > This is fine by me, however:
    > > 1. Can you add some comments to say "this is not for core vm but for pat,
    > > oh and a pgoff of zero is not going to work".
    >
    > OK. Will add comments about both the points.
    >
    > > 2. Can you please justify to me (or the changelog) roughly why PAT wants
    > > to know if the mapping is linear or not? Presumably it has to handle
    > > both types? If performance wasn't an issue, then you could manually scan
    > > the ptes to verify (which would solve your zero-offset bug). etc.
    >
    > The main reason is performance. If we know it is linear, we can track the entire
    > region as one block and do the reserve free for entire region. But, if it is
    > not linear, then we have to reserve memtype of physical addresses page by page.
    > This will not be optimal as it will result in reserve and free becoming
    > slower. Almost all users that we find in kernel today (atleast in x86) are
    > all linear.

    OK, so it is not a bug to miss the zero pgoff case then. That's good
    to know and should be added to comments.



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