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    SubjectRe: [PATCH][5/?] count writeback pages in nr_scanned
    On Wed, Jan 05, 2005 at 09:37:04PM -0800, Andrew Morton wrote:
    > Andrea Arcangeli <andrea@suse.de> wrote:
    > >
    > > 2) we won't need unreliable anti-deadlock timeouts anymore
    >
    > The timeouts are for:
    >
    > a) A fallback for backing stores which don't wake up waiters in
    > blk_congestion_wait() (eg: NFS).

    that anti-deadlock will be unnecessary too with the new logic.

    > b) handling the race case where the request queue suddenly goes empty
    > before the sleeper gets onto the waitqueue.

    as I mentioned with proper locking setting task in uninterruptible and
    then registering into the new per classzone waitqueue, the timeout will
    be unnecessary even for this.

    > It can probably be removed with some work, and additional locking.

    The additional locking will then remove the current locking in
    blk_congestion_wait so it's new locking but it will replace the current
    locking. But I agree registering in the waitqueue inside the
    blk_congestion_wait was simpler. It's just I've an hard time to like the
    timeout. Timeout is always wrong when it triggers: if it triggers it
    always triggers either too late (wasted resources) or too early (early
    oom kills). So unless it messes everything up, it'd be nice to the
    locking strict. anyway point 1 and 2 can be implemented separately, at
    first we can leave the timeout if the race is too hard to handle.

    Ideally if we keep the total number of oustanding writebacks
    per-classzone (not sure if we account for it already somewhere, I guess
    if something we've the global number and not the per-classzone one), we
    could remove the timeout without having to expose the locking outside
    blk_congestion_wait. With the number of oustanding writebacks
    per-classzone we could truly fix the race and obsolete the timeout in a
    self contained manner. Though it will require a proper amount of memory
    barriers around the account increment/decrement/read.
    -
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