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 10:11:39AM -0800, Darrick J. Wong wrote:
> I think this has the same problem that Dave complained about back in
> October -- if I have a hard disk with 512b sectors, a xfs with 4k
> blocks, and mount it on a machine with 64k pages, then single-sector
> buffers will get a small slab allocation, but single-fsb buffers will
> now fall back to grabbing a 64k page to hold 4k worth of data.
> 
> Even if we add a second slab for single-fsb blocks, we'll run into the
> same problem if the filesystem contains multi-block directory blocks, so
> then we'd potentially need a third slab, and... is there a way to ask a
> slab allocator for multiple contiguous objects totalling less than
> PAGE_SIZE bytes?

No, unfortunately there is not.  And the sad part is that generally it
will give you aligned ones, just with slub debugging turned on it won't.