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

From: Matthew Wilcox <willy@infradead.org>
To: Ming Lei <tom.leiming@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, Ming Lei <ming.lei@redhat.com>,
	Dave Chinner <david@fromorbit.com>,
	"Darrick J . Wong" <darrick.wong@oracle.com>,
	"open list:XFS FILESYSTEM" <linux-xfs@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Dave Chinner <dchinner@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	Aaron Lu <aaron.lu@intel.com>, Christopher Lameter <cl@linux.com>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	linux-block <linux-block@vger.kernel.org>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc
Date: Tue, 26 Feb 2019 04:12:09 -0800	[thread overview]
Message-ID: <20190226121209.GC11592@bombadil.infradead.org> (raw)
In-Reply-To: <CACVXFVMX=WpTRBbDTSibfXkTZxckk3ootetbE+rkJtHhsZkRAw@mail.gmail.com>

On Tue, Feb 26, 2019 at 07:12:49PM +0800, Ming Lei wrote:
> 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.

Why does the block size make a difference?  This requirement is due to
some storage devices having shoddy DMA controllers.  Are you saying there
are devices which can't even do 512-byte aligned I/O?