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

[Ming Lei <ming.lei@canonical.com>]

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@iki.fi>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Simon Baatz <gmbnomis@gmail.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Tejun Heo <tj@kernel.org>
Cc: "James E.J. Bottomley" <JBottomley@parallels.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 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))
 			flush_kernel_dcache_page(miter->page);
 
 		if (miter->__flags & SG_MITER_ATOMIC) {
-- 
1.7.9.5