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

[Simon Baatz]

On Tue, Oct 29, 2013 at 12:20:05AM +0800, Ming Lei wrote:
> 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 at iki.fi>
> Cc: Russell King - ARM Linux <linux at arm.linux.org.uk>
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: Simon Baatz <gmbnomis at gmail.com>
> Cc: Will Deacon <will.deacon at arm.com>
> Cc: Aaro Koskinen <aaro.koskinen at iki.fi>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> 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: Ming Lei <ming.lei 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 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
> 

Tested-by: Simon Baatz <gmbnomis at gmail.com>

on top of 3.12-rc7 on Kirkwood with DEBUG_VM enabled.

- Simon