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    SubjectRe: [PATCH block:for-3.3/core] cfq: merged request shouldn't jump to a different cfqq
    On Thu, 2012-01-05 at 18:36 -0800, Tejun Heo wrote:
    > Hello, again.
    > On Thu, Jan 05, 2012 at 06:17:07PM -0800, Tejun Heo wrote:
    > > When two requests are merged, if the absorbed request is older than
    > > the absorbing one, cfq_merged_requests() tries to reposition it in the
    > > cfqq->fifo list by list_move()'ing the absorbing request to the
    > > absorbed one before removing it.
    > >
    > > This works if both requests are on the same cfqq but nothing
    > > guarantees that and the code ends up moving the merged request to a
    > > different cfqq's fifo list without adjusting the rest. This leads to
    > > the following failures.
    > >
    > > * A request may be on the fifo list of a cfqq without holding
    > > reference to it and the cfqq can be freed before requst is finished.
    > > Among other things, this triggers list debug warning and slab debug
    > > use-after-free warning.
    > >
    > > * As a request can be on the wrong fifo queue, it may be issued and
    > > completed before its cfqq is scheduled. If the cfqq didn't have
    > > other requests on it, it would be empty by the time it's dispatched
    > > triggering BUG_ON() in cfq_dispatch_request().
    > >
    > > Fix it by making cfq_merged_requests() scan the absorbing request's
    > > fifo list for the correct slot and move there instead.
    > Hmmm... while the patch would fix the problem. It isn't entirely
    > correct. The root cause is,
    > 1. q->last_merge and rqhash used to be used only for merging bios into
    > requests and that queries elevator whether the merge should be
    > allowed. cfq disallows merging if they belong to different cfqqs.
    > 2. request-request merging didn't use to use q->last_merge or rqhash to
    > find request candidates. It used elv_former/latter_request() and
    > cfq never returned request from a different cfqq.
    > 3. Plug merging started using q->last_merge and rqhash and now
    > elevator can't prevent cross cfqq merges.
    > So, yeah, the right fix would be using elv_former/latter_request()
    > instead. Maybe we should strip out rqhash altogether and change
    > elevator handle everything? I don't know. I'll prepare a different
    > fix patch soon.
    So not allow merge from two cfq queues strictly? This will impact
    performance. I don't know how important the strict isolation is. we even
    allow two cfq queues merge to improve performance.

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