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    SubjectRe: [PATCH] mmc-atmel : fix kunmap wrong usage
    On Fri, 22 Jun 2007, Marc Pignat wrote:
    > from: Marc Pignat <marc.pignat@hevs.ch>
    >
    > kunmap must be called on the pointer returned by kmap.

    Not necessarily: an offset within the same page is acceptable.

    >
    > Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
    >
    > ---
    >
    > N.B: This is the same patch as yesterday, with proper Signed-off-by and more
    > comments.
    >
    > The buffer variable is used this way:
    > buffer = kmap_atomic(sg->page, KM_BIO_SRC_IRQ) + sg->offset;
    > So we can't do
    > kunmap_atomic(buffer, KM_BIO_SRC_IRQ);
    > we must do
    > kunmap_atomic(buffer - sg->offset, KM_BIO_SRC_IRQ);
    >
    > Strangely this misuse showed no problem using CONFIG_SLAB, but oops using
    > CONFIG_SLUB, (lkml ''Oops in a driver while using SLUB as a SLAB allocator").

    Aren't you just guessing there? Those kunmap_atomics in at91_mci.c
    may look wrong to you, but they're not incorrect (so long as sg->offset
    falls within the page, as it must do here to make sense of the page).
    Especially not on ARM, where kunmap_atomic actually has no interest
    in the argument passed. And the oops was in the flush_dcache_page.

    If you actually reproduced Nicolas' problem on ARM, and verified
    that your patch then fixes it, please let us know: that will be
    remarkably interesting.

    I believe I posted the correct fix last night (or at least a safe fix
    for now: Christoph Lameter may prefer to undo it and change ARM's
    dma_mapping at leisure later), checking PageSlab in page_mapping;
    and Linus has already put that one into his git tree.

    But it would be good to hear from Nicolas whether that indeed fixes
    his oops: I couldn't actually try my patch on ARM either.

    Hugh

    >
    > Regards
    >
    > Marc
    >
    > --- drivers/mmc/host/at91_mci.c-2.6.22-rc5.orig 2007-06-21 16:27:31.000000000 +0200
    > +++ drivers/mmc/host/at91_mci.c-2.6.22-rc5 2007-06-21 16:42:48.000000000 +0200
    > @@ -164,7 +164,7 @@ static inline void at91mci_sg_to_dma(str
    > else
    > memcpy(dmabuf, sgbuffer, amount);
    >
    > - kunmap_atomic(sgbuffer, KM_BIO_SRC_IRQ);
    > + kunmap_atomic(sgbuffer - sg->offset, KM_BIO_SRC_IRQ);
    >
    > if (size == 0)
    > break;
    > @@ -293,7 +293,7 @@ static void at91mci_post_dma_read(struct
    > buffer[index] = swab32(buffer[index]);
    > }
    >
    > - kunmap_atomic(buffer, KM_BIO_SRC_IRQ);
    > + kunmap_atomic(buffer - sg->offset, KM_BIO_SRC_IRQ);
    > flush_dcache_page(sg->page);
    -
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