[PATCH] bounce:fix bug, avoid to flush dcache on slab page from jbd2.

[akpm@linux-foundation.org (Andrew Morton)]

On Wed, 13 Mar 2013 11:35:15 +0800 Shuge <shugelinux@gmail.com> wrote:

> Hi all
> >>> The bounce accept slab pages from jbd2, and flush dcache on them.
> >>> When enabling VM_DEBUG, it will tigger VM_BUG_ON in page_mapping().
> >>> So, check PageSlab to avoid it in __blk_queue_bounce().
> >>>
> >>> Bug URL: http://lkml.org/lkml/2013/3/7/56
> >>>
> >>> ...
> >>>
> >> ......
> >>
> > That sure is strange.  I didn't see any obvious reasons why we'd end up with a
> >
> ......
> 
>      Well, this problem not only appear in arm64, but also arm32. And my 
> kernel version is 3.3.0, arch is arm32.
> Following the newest kernel, the problem shoulde be exist.
>      I agree with Darrick's modification. Hum, if 
> CONFIG_NEED_BOUNCE_POOL is not set, it also flush dcahce on
> the pages of b_frozen_data, some of them are allocated by kmem_cache_alloc.
>      As we know, jbd2_alloc allocate a buffer from jbd2_xk slab pool, 
> when the size is smaller than PAGE_SIZE.
> The b_frozen_data  is not mapped to usrspace, not aliasing cache. It cat 
> be lazy flush or other. Is it right?

Please reread my email.  The page at b_frozen_data was allocated with
GFP_NOFS.  Hence it should not need bounce treatment (if arm is
anything like x86).

And yet it *did* receive bounce treatment.  Why?