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    SubjectRe: [patch 3/3] mm: fault handler to replace nopage and populate
    On Mon, Oct 09, 2006 at 10:07:50PM +1000, Benjamin Herrenschmidt wrote:
    > On Mon, 2006-10-09 at 13:58 +0200, Nick Piggin wrote:
    > >
    > > The VM won't see that you have struct pages backing the ptes, and won't
    > > do the right refcounting or rmap stuff... But for file backed mappings,
    > > all the critical rmap stuff should be set up at mmap time, so you might
    > > have another option to simply always do the nopfn thing, as far as the
    > > VM is concerned (ie. even when you do have a struct page)
    >
    > Any reason why it wouldn't work to flip that bit on the first no_page()
    > after a migration ? A migration always involves destroying all PTEs and
    > is done with a per-object mutex held that no_page() takes too, so we can
    > be pretty sure that the first nopage can set that bit before any PTE is
    > actually inserted in the mapping after all the previous ones have been
    > invalidated... That would avoid having to walk the vma's.

    Ok I guess that would work. I was kind of thinking that one needs to
    hold the mmap_sem for writing when changing the flags, but so long
    as everyone *else* does, then I guess you can get exclusion from just
    the read lock. And your per-object mutex would prevent concurrent
    nopages from modifying it.
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