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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Dec 06, 2018 at 09:11:06PM +0100, Christoph Hellwig wrote:
> 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.

Hmmm.  I guess we could declare three slabs then -- sector size, fsb size,
and dir fsb size?  This is getting kinda spidery now though.  Is three
too weird?

--D