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    SubjectRe: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and _NONVOLATILE flags
    On Wed, 2012-02-15 at 10:51 +1100, Dave Chinner wrote:
    > Given that it is a single state bit per page (volatile/non volatile)
    > you could just use a radix tree tag for keeping the state. Changing
    > the state isn't a performance critical operation, and tagging large
    > ranges isn't that expensive (e.g. we do that in the writeback code),
    > so I'm not sure the overhead of a separate tree is necessary here....

    Hrm. I'll look into this.

    > That doesn't help with the reclaim side of things, but I would have
    > thought that such functioanlity would be better integrated into the
    > VM page cache/lru scanning code than adding a shrinker to shrink the
    > page cache additionally on top of what the VM has already done
    > before calling the shrinkers. I'm not sure what is best here,
    > though...

    Yea. My previous version did eviction from shmem_writepage(), I believe
    much as you suggest here, but the concern with that is that you could
    have a larger volatile range that was for the most part recently in use,
    but one idle page causes the entire thing to be evicted first. Using
    least-recently-marked-volatile order seems more natural for the use case
    (although Dimitry and others have already pointed out that the
    inheritance from the coalescing of neighboring ranges results in a
    similar issue).

    But I'm open to other ideas and arguments.

    Thanks again for the feedback!

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