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

Dave Chinner
2019-02-26T03:02:19Z

On Tue, Feb 26, 2019 at 10:22:50AM +0800, Ming Lei wrote:
> On Tue, Feb 26, 2019 at 07:26:30AM +1100, Dave Chinner wrote:
> > On Mon, Feb 25, 2019 at 02:15:59PM +0100, Vlastimil Babka wrote:
> > > On 2/25/19 5:36 AM, Dave Chinner wrote:
> > > > On Mon, Feb 25, 2019 at 12:09:04PM +0800, Ming Lei wrote:
> > > >> XFS uses kmalloc() to allocate sector sized IO buffer.
> > > > ....
> > > >> Use page_frag_alloc() to allocate the sector sized buffer, then the
> > > >> above issue can be fixed because offset_in_page of allocated buffer
> > > >> is always sector aligned.
> > > >
> > > > Didn't we already reject this approach because page frags cannot be
> > > > reused and that pages allocated to the frag pool are pinned in
> > > > memory until all fragments allocated on the page have been freed?
> > >
> > > I don't know if you did, but it's certainly true., Also I don't think
> > > there's any specified alignment guarantee for page_frag_alloc().
> >
> > We did, and the alignment guarantee would have come from all
> > fragments having an aligned size.
> >
> > > What about kmem_cache_create() with align parameter? That *should* be
> > > guaranteed regardless of whatever debugging is enabled - if not, I would
> > > consider it a bug.
> >
> > Yup, that's pretty much what was decided. The sticking point was
> > whether is should be block layer infrastructure (because the actual
> > memory buffer alignment is a block/device driver requirement not
> > visible to the filesystem) or whether "sector size alignement is
> > good enough for everyone".
>
> OK, looks I miss the long life time of meta data caching, then let's
> discuss the slab approach.
>
> Looks one single slab cache doesn't work, given the size may be 512 * N
> (1 <= N < PAGE_SIZE/512), that is basically what I posted the first
> time.
>
> https://marc.info/?t=153986884900007&r=1&w=2
> https://marc.info/?t=153986885100001&r=1&w=2
>
> 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.

> example, if 99% times falls in 512 byte allocation, maybe it is enough
> to just maintain one 512byte slab.

It is not. On a 64k page size machine, we use sub page slabs for
metadata blocks of 2^N bytes where 9 <= N <= 15..

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com