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

Dave Chinner
2019-02-25T20:28:20Z

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".

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com