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    SubjectRe: [patch] block: properly handle flush/fua requests in blk_insert_cloned_request

    On Tue, Aug 09, 2011 at 11:53:51AM -0400, Jeff Moyer wrote:
    > Tejun Heo <> writes:
    > > I'm a bit confused. We still need ELEVATOR_INSERT_FLUSH fix for
    > > insertion paths, right? Or is blk_insert_cloned_request() supposed to
    > > used only by request based dm which lives under the elevator? If so,
    > > it would be great to make that explicit in the comment. Maybe just
    > > renaming it to blk_insert_dm_cloned_request() would be better as it
    > > wouldn't be safe for other cases anyway.
    > request-based dm is the only caller at present. I'm not a fan of
    > renaming the function, but I'm more than willing to comment it.

    I'm still confused and don't think the patch is correct (you can't
    turn off REQ_FUA without decomposing it to data + post flush).

    Going through flush machinery twice is okay and I think is the right
    thing to do. At the upper queue, the request is decomposed to member
    requests. After decomposition, it's either REQ_FLUSH w/o data or data
    request w/ or w/o REQ_FUA. When the decomposed request reaches lower
    queue, the lower queue will then either short-circuit it, execute
    as-is or decompose data w/ REQ_FUA into data + REQ_FLUSH sequence.

    AFAICS, the breakages are...

    * ELEVATOR_INSERT_FLUSH not used properly from insert paths.

    * Short circuit not kicking in for the dm requests. (the above and the
    policy patch should solve this, right?)

    * BUG(!rq->bio || ...) in blk_insert_flush(). I think we can lift
    this restriction for empty REQ_FLUSH but also dm can just send down
    requests with empty bio.



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