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    SubjectRe: [patch 5/18] mark swapout pages PageWriteback()
    On Fri, 31 May 2002, Linus Torvalds wrote:
    > On Fri, 31 May 2002, Andrea Arcangeli wrote:
    > >
    > > In short the same way MAP_ANON must pageout correctly, also MAP_SHARED
    > > must swapout correctly with very vm intensive conditions.

    I strongly agree with Andrea on that. Swap fault without page lock
    was a step forward, reinstating it in the light of observed degradation
    in one workload was right decision for that particular tail of release,
    but it's a shame to have stayed that way.

    > That is true, but it is ignoring the fact that there _are_ real technical
    > differences between swap cache mappings and regular shared mappings.
    > One major difference is the approach to the last user: a last use of a
    > shared mapping still needs to write out dirty state, while the last use of
    > a swap page is better off noticing that it should just optimize away the
    > write, and we can just turn the page back into a dirty anonymous page.

    Poor example: isn't last use of swap page just like last use of
    shared mapping of an _unlinked_ file?

    Offhand, I think most of the differences between swap and filesystem
    just come from our wish not to waste any time on that filesystem:
    we _expect_ the object will be unlinked before it needs to hit disk.
    Plus, it's important that going to disk doesn't need more memory.

    Perhaps those are just attributes of a particular filesystem.
    Just as anon pages graduated to being put on LRU a few months ago,
    maybe they could graduate to being hashed pages of tmpfs objects?
    Probably yes, but without wasting time and memory - more doubtful.


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