[PATCH 3.5 33/78] lib/scatterlist.c: don't flush_kernel_dcache_page on slab page

[Luis Henriques]

3.5.7.26 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ming Lei <ming.lei at canonical.com>

commit 3d77b50c5874b7e923be946ba793644f82336b75 upstream.

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().

Signed-off-by: Ming Lei <ming.lei at canonical.com>
Reported-by: Aaro Koskinen <aaro.koskinen at iki.fi>
Tested-by: Simon Baatz <gmbnomis at gmail.com>
Cc: Russell King - ARM Linux <linux at arm.linux.org.uk>
Cc: Will Deacon <will.deacon at arm.com>
Cc: Aaro Koskinen <aaro.koskinen at iki.fi>
Acked-by: Catalin Marinas <catalin.marinas at arm.com>
Cc: FUJITA Tomonori <fujita.tomonori at lab.ntt.co.jp>
Cc: Tejun Heo <tj at kernel.org>
Cc: "James E.J. Bottomley" <JBottomley at parallels.com>
Cc: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 lib/scatterlist.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 6096e89..8c2f278 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -419,7 +419,8 @@ void sg_miter_stop(struct sg_mapping_iter *miter)
 	if (miter->addr) {
 		miter->__offset += 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) {
-- 
1.8.3.2