[PATCH] lib/scatterlist.c: don't flush_kernel_dcache_page on slab page

From: Ming Lei
Date: Mon Oct 28 2013 - 12:20:36 EST

Commit b1adaf65ba03([SCSI] block: add sg buffer copy helper functions)
introduces two sg buffer copy helpers, and calls flush_kernel_dcache_page()
on pages in SG list after these pages are written to.

Unfortunately, the commit may introduce a potential bug:

- Before sending some SCSI commands, kmalloc() buffer may be
passed to block layper, so flush_kernel_dcache_page() can
see a slab page finally

- According to cachetlb.txt, flush_kernel_dcache_page() is
only called on "a user page", which surely can't be a slab page.

- ARCH's implementation of flush_kernel_dcache_page() may
use page mapping information to do optimization so page_mapping()
will see the slab page, then VM_BUG_ON() is triggered.

Aaro Koskinen reported the bug on ARM/kirkwood when DEBUG_VM is enabled,
and this patch fixes the bug by adding test of '!PageSlab(miter->page)'
before calling flush_kernel_dcache_page().

Reported-by: Aaro Koskinen <aaro.koskinen@xxxxxx>
Cc: Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: Simon Baatz <gmbnomis@xxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Aaro Koskinen <aaro.koskinen@xxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: "James E.J. Bottomley" <JBottomley@xxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
lib/scatterlist.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index a685c8a..d16fa29 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -577,7 +577,8 @@ void sg_miter_stop(struct sg_mapping_iter *miter)
miter->__offset += miter->consumed;
miter->__remaining -= miter->consumed;

- if (miter->__flags & SG_MITER_TO_SG)
+ if ((miter->__flags & SG_MITER_TO_SG) &&
+ !PageSlab(miter->page))

if (miter->__flags & SG_MITER_ATOMIC) {

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