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

Ming Lei
2019-02-26T02:24:51Z

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

Thanks,
Ming