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    SubjectRe: [PATCH] dma-debug: disable DMA_API_DEBUG for now

    * Joerg Roedel <> wrote:

    > On Fri, Jun 05, 2009 at 05:33:14PM +0900, FUJITA Tomonori wrote:
    > > dma-debugs wrongly assumes that no multiple DMA transfers are
    > > performed against the same dma address on one device at the same
    > > time. However it's true only with hardware IOMMUs. For example, an
    > > application can easily send the same buffer twice with different
    > > lengths to one device by using DIO and AIO. If these requests are not
    > > unmapped in the same order in which they were mapped,
    > > hash_bucket_find() finds a wrong entry and gives a false warning.
    > >
    > > We should fix this before 2.6.30 release. Seems that there is no
    > > easy way to fix it. I think that it's better to just disable
    > > dma-debug for now.
    > >
    > > Torsten Kaiser found this bug with the RAID1 configuration:
    > >
    > >
    > >
    > Argh, I never thought that this can happen. But its not explicitly
    > forbidden so we have to handle this situation. Thanks for tracking
    > down the bug to this issue.
    > However, I think there is a somehow simple fix for the issue.
    > Patch is attached. Its the least intrusive way I can think of to
    > fix this problem.
    > But its up to Linus/Ingo to decide if it can be accepted at this
    > very late point in the cycle. Since dma-debug is new with 2.6.30
    > it will at least not introduce any regression. [...]

    I think it's too late for v2.6.30 to do any of the changes - and the
    DMA debug facility is off by default.

    Also, i think such DMA patterns, while 'allowed' can be quite
    dangerous as its such a rare usage combination really. AIO and DIO
    are crazy to begin with, mixing AIO and DIO for the same buffer is
    madness square two. (It can result in 3 agents for the same memory
    address: CPU, dma1 and dma2. How many interesting chipset erratums
    could there be related to such scenarios?)

    But it is certainly not the task of a debug facility to restrict
    existing user-visible ABIs, so fixing the false positive is correct.

    So i've applied your fix to the iommu branch for v2.6.31 and marked
    it for -stable backporting, that way will be able to pick
    the patch up (if it remains problem-free in testing).



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