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    SubjectRe: [PATCH 09/11] unpaged: ZERO_PAGE in VM_UNPAGED
    On Thu, 17 Nov 2005, Ingo Oeser wrote:
    >
    > We do we refcount ZERO_PAGE at all?

    We never used to. They were, and for the moment still are, marked
    PageReserved. Prior to 2.6.15-rc we didn't refcount reserved pages,
    but now we're trying to move away from PageReserved (some differences
    of opinion how far to go), so refcounting them.

    We're currently refcounting the ZERO_PAGE(s) simply because the
    common case is refcounted: it would just be extra tests and code
    NOT to refcount the ZERO_PAGE(s). If they were a commoner case,
    then it would indeed be worth avoiding refcounting them, but it
    currently doesn't appear to be worth the effort.

    But it is still up in the air: there is or may be an issue with
    refcounts overflowing, and if it's clear that the ZERO_PAGE is
    the only one vulnerable on any architecture, then I'm sure we'd
    deal with it by not refcounting them. However, I believe the
    issue extends to mapped file pages too: though you need a huge
    amount of RAM to reach overflow, so it's not something we need
    to resolve this week.

    > Ok, there may be multiple, but they exist always and always at
    > the same physical addresses, right?

    Right.

    > So why do we care at all?
    > Memory hotplug?
    > Doesn't it suffice there, that they are reverse mappable?

    Actually, they're not reverse mappable: we tend to find
    there's not much gained by swapping out the ZERO_PAGE ;)

    Hugh
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