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

Ming Lei
2019-02-26T11:14:48Z

On Tue, Feb 26, 2019 at 6:07 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 2/26/19 10:33 AM, Ming Lei wrote:
> > 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.
> >
>
> Where does the '*4' multiplier come from?

The buffer needs to be device block size aligned for dio, and now the block
size can be 512, 1024, 2048 and 4096.

Thanks,
Ming Lei