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@xxxxxxxxxxxxx
