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    SubjectRe: [PATCH][RFC] splice support
    On Thu, Mar 30 2006, Nick Piggin wrote:
    > Hi Jens,
    > Looks nice!
    > Jens Axboe wrote:
    > >Hi,
    > >
    > >Since my initial posting back in December, I've had some private queries
    > >about the state of splice support. The state was pretty much that it was
    > >a little broken, if one attempted to do file | file splicing. The
    > >original patch migrated pages from one file to another in this case,
    > >which got vm ugly really quickly. And it wasn't always the right thing
    > >to do, since it would mean that splicing file1 to file2 would move
    > >file1's page cache to file2. Sometimes this is what you want, sometimes
    > >it is
    > >
    > Page migration now generalised vmscan.c and introduced remove_mapping
    > function, which should help keep things clean.


    > Moving a page onto and off the LRU is an interesting problem, though.
    > But possibly you could just leave it on the LRU and transfer the pagecache
    > reference over to the pipe. vmscan would find extra pages on the LRU at
    > times, but they would go away when pipe releases the page.
    > Moving a page from a pipe to a filesystem might be harder, because you
    > don't know if it came from a filesystem (still on LRU) or not (in which
    > case you need to add it to LRU). If only you can keep track of this

    Well that, to me, is _the_ hard problem to solve for this. But you
    sort-of do know, my plan is/was to add a ->steal() hook to the pipe
    buffers that would 'unhook' the page so it was in a clean state to be
    added to the LRU/page cache again. If stealing failed, just fall back to
    copying (or hard error, let the flags decide).

    > information as the page gets passed around... hmm the PG_private will be
    > free to use because a filesystem must always drop its buffers before
    > remove_mapping can run. One would also need to take care of replacing
    > an existing page I guess.
    > Hmm... I think it can work, falling back to copies if we get stuck
    > anywhere.
    > Unless someone beats me to it, I'll try coding something up when I get
    > a bit more free time.

    You are more than welcome, I hope to give it a little shot today and see
    how it goes.

    Jens Axboe

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