Re: [PATCH v2] xfs: use a dedicated SLAB cache for sector sized buffer data

[Christoph Hellwig <hch@lst.de>]

On Thu, Dec 06, 2018 at 07:51:52AM -0500, Brian Foster wrote:
> On Wed, Dec 05, 2018 at 02:51:47PM -0800, Christoph Hellwig wrote:
> > XFS currently uses kmalloc for buffers smaller than the page size to
> > avoid wasting too much memory.  But this can cause a problem with slub
> > debugging turned on as the allocations might not be naturally aligned.
> > On block devices that require sector size alignment this can lead to
> > data corruption.
> > 
> > Give that our smaller than page size buffers are always sector sized
> > on a live file system, we can just create a kmem_cache with an
> > explicitly specified alignment requirement for this case to fix this
> > case without much effort.
> > 
> 
> What exactly is the data corruption related problem? Can you
> characterize it in a couple sentences?

Ming reported the actual occurance, so he can explain in detail.  But
the summary is that various devices require a minimum alignment for DMA,
and if we don't follow that bad things will happen.  What "bad things"
are might vary from case to case.