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    SubjectRe: [patch 1/9] Guest page hinting: unused / free pages.
    Martin Schwidefsky wrote:
    > On Wed, 2006-09-13 at 10:45 -0400, Rik van Riel wrote:
    >
    >>>But another trouble you have not mentioned is what happens to pages
    >>>with pending make-volatile that need to and/or have been made stable
    >>>in the meantime. They too need to be removed from this pending list.
    >>
    >>At the time where you walk the set of pages (pagevec?) to make
    >>volatile, you can check whether the page flags are still right.
    >
    >
    > A make volatile can be done anytime as long as the page is in page
    > cache. Before a page can be made stable the caller needs to make sure
    > that one of the conditions that prevent a make volatile becomes true.
    > So a page in the pending make-volatile array does not have to be removed
    > because a make stable has been done. It only has to be removed if it
    > gets freed.
    >
    >
    >>A page that was set to be marked volatile with the hypervisor,
    >>but later turned stable again would have that indicated in its
    >>page flags, right?
    >
    >
    > Several page flag bits and some other conditions like "has a mapping"
    > and "reference count is map count + 1". Most of the checks that need to
    > be done for make volatile are on page flags.
    >

    Interesting..
    But don't we have to do some locking on the page to avoid race conditions?
    A page needs to be consistent between check through __page_discardable and
    committing to the hypervisor. We could raise the PG_state_change flag for
    that period.


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