Re: [PATCH] mmc-atmel : fix kunmap wrong usage

On Fri, 22 Jun 2007, Marc Pignat wrote:
from: Marc Pignat <marc.pignat@xxxxxxx>

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@xxxxxxx>

---

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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