On Tue, Feb 26, 2019 at 04:12:09AM -0800, Matthew Wilcox wrote: > 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@xxxxxxx> 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? Direct IO requires that, see do_blockdev_direct_IO(). This issue can be triggered when running xfs over loop/dio. We could fallback to buffered IO under this situation, but not sure it is the only case. Thanks, Ming
