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

[Christoph Hellwig <hch@lst.de>]

On Thu, Dec 06, 2018 at 08:50:00AM +1100, Dave Chinner wrote:
> On Wed, Dec 05, 2018 at 09:20:23AM -0800, Christoph Hellwig wrote:
> > XFS currently uses a slab allocator for buffers smaller than the page
> 
> Currently uses heap allocated memory?

Or kmalloc, yeah.

> I thought you were looking at using the page_frag infrastructure for
> this so we didn't need a slab per filesystem for this?

I did look into it, but using page_frag for buffers creates an inherent
memory leak given that page_frag never reuses a freed fragement.  So
until all fragments in a page are free all the others bits effectively
leak, which is pretty bad on the buffer cache.

> I think there's a problem with this code - we can have different
> sector sizes for different devices in the filesystem. e.g. the log
> can have a different sector size to the data device, and this may
> matter for log recovery which does a lot of single sector IO.

That's fine because log recovery is one I/O at a time, and we always
free the current buffer before allocating the next one, so we'd waste
the memory one way or another.

> Hence I think this slab really needs to be managed as a property of
> the of the buftarg rather than be a global property of the
> xfs_mount. in most cases the log and data devices are the same
> buftarg, but we should attempt to handle this special case
> correctly...

We could waste another slab cache on the block device, but as said
we don't really save any memory by creating a new cache, we actually
waste memory due to the overhead.