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    SubjectRe: 2.4 and 2GB swap partition limit

    On Tue, May 01, 2001 at 06:14:54PM +0200, Rogier Wolff wrote:

    > Shouldn't the algorithm be:
    > - If (current_access == write )
    > free (swap_page);
    > else
    > map (page, READONLY)
    > and
    > when a write access happens, we fault again, and map free the
    > swap-page as it is now dirty anyway.

    That's what 2.2 did. 2.4 doesn't have to.

    The trouble is, you really want contiguous virtual memory to remain
    contiguous on swap. Freeing individual pages like this on fault can
    cause a great deal of fragmentation in swap. We'd far rather keep the
    swap page reserved for future use by the same page so that the VM
    region remains contiguous on disk.

    That's fine as far as it goes, but the problem happens if you _never_
    free up such pages. We should reap the unused swap page if we run out
    of swap. We don't, and _that_ is the problem --- not the fact that
    the page is left allocated in the first place, but the fact that we
    don't do anything about it once we are short on disk.

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