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    SubjectRe: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments about VMERGE
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    On Tue, 15 Jul 2008 10:37:58 -0400 (EDT)
    Mikulas Patocka <mpatocka@redhat.com> wrote:

    > >>> This bug could happen on alpha, parisc, and sparc, which use VMERGE.
    > >>
    > >> Parisc doesn't use virtual merge accounting (there is variable for it but
    > >> it's always 0).
    > >
    > > Hmm, really? Looks like PARISC IOMMUs (ccio-dma.c and sba_iomm.c) set
    > > parisc_vmerge_boundary (CC'ed PARISC mailing list).
    >
    > That's right, I looked only at arch and include.
    >
    > >> On sparc64 it is broken anyway with or without your patch.
    > >
    > > Yeah, we need to modify SPARC64 IOMMU code (I'm not sure that it's
    > > worth). Right now, the best fix is setting BIO_VMERGE_BOUNDARY to 0.
    >
    > Even if we fix it now, the question is: how long it will stay fixed? Until
    > someone makes another change to struct device that restricts boundaries on
    > some wacky hardware.

    I'm not sure how the boundary restriction of a device can break
    the VMERGE accounting.


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