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    SubjectRe: [patch] 'virtual => physical page mapping cache', vcache-2.5.38-B8

    On Fri, 27 Sep 2002, Linus Torvalds wrote:

    > You may want to do something clever to avoid taking the vcache lock in
    > the COW path unless there is real reason to believe that you have to,
    > but as far as I can tell, you can do that by simply adding a memory
    > barrier to the end of the "insert vcache entry" thing.

    the COW path lookup code already avoids taking the vcache lock - unless
    the hash-head is non-empty.

    > Because once you do that, and you make sure that the COW thing does the
    > vcache callback _after_ changing the page tables, the COW code can
    > - do the hash (which is invariant and has no races, since it depends
    > solely on the virtual address and the VM)
    > - check if the hash queue is empty
    > - only if the hash queue is non-empty does the COW code need to get the
    > vcache lock (and obviously it needs to re-load the hash entry from the
    > queue after it got the lock, but the hash is still valid)

    yeah, this is the optimization i have described in the previous mail, and
    which is in the patch.

    > Maybe I'm missing something, but the locking really doesn't look all
    > that problematic if we just do things in the obvious order..

    i agree, first hashing the vcache should work. There are some details:
    if we first hash the vcache then we have to set up the queue in a way for
    the callback function to notice that this queue is not futex-hashed (ie.
    not live) yet. Otherwise the callback function might attempt to rehash it.
    This means one more branch in the callback function, not a problem.


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