Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc

[Ming Lei <ming.lei@redhat.com>]

On Tue, Feb 26, 2019 at 03:58:26PM +1100, Dave Chinner wrote:
> On Mon, Feb 25, 2019 at 07:27:37PM -0800, Matthew Wilcox wrote:
> > On Tue, Feb 26, 2019 at 02:02:14PM +1100, Dave Chinner wrote:
> > > > Or what is the exact size of sub-page IO in xfs most of time? For
> > > 
> > > Determined by mkfs parameters. Any power of 2 between 512 bytes and
> > > 64kB needs to be supported. e.g:
> > > 
> > > # mkfs.xfs -s size=512 -b size=1k -i size=2k -n size=8k ....
> > > 
> > > will have metadata that is sector sized (512 bytes), filesystem
> > > block sized (1k), directory block sized (8k) and inode cluster sized
> > > (32k), and will use all of them in large quantities.
> > 
> > If XFS is going to use each of these in large quantities, then it doesn't
> > seem unreasonable for XFS to create a slab for each type of metadata?
> 
> 
> Well, that is the question, isn't it? How many other filesystems
> will want to make similar "don't use entire pages just for 4k of
> metadata" optimisations as 64k page size machines become more
> common? There are others that have the same "use slab for sector
> aligned IO" which will fall foul of the same problem that has been
> reported for XFS....
> 
> If nobody else cares/wants it, then it can be XFS only. But it's
> only fair we address the "will it be useful to others" question
> first.....

This kind of slab cache should have been global, just like interface of
kmalloc(size).

However, the alignment requirement depends on block device's block size,
then it becomes hard to implement as genera interface, for example:

	block size: 512, 1024, 2048, 4096
	slab size: 512*N, 0 < N < PAGE_SIZE/512

For 4k page size, 28(7*4) slabs need to be created, and 64k page size
needs to create 127*4 slabs.

But, specific file system may only use some of them, and it depends
on meta data size.

Thanks,
Ming