On Mon, Sep 24, 2018 at 12:09:37PM -0600, Jens Axboe wrote: > On 9/24/18 12:00 PM, Christopher Lameter wrote: > > On Mon, 24 Sep 2018, Jens Axboe wrote: > > > >> The situation is making me a little uncomfortable, though. If we export > >> such a setting, we really should be honoring it... That's what I said up front, but you replied to this with: | I think this is all crazy talk. We've never done this, [...] Now I'm not sure what you are saying we should do.... > > Various subsystems create custom slab arrays with their particular > > alignment requirement for these allocations. > > Oh yeah, I think the solution is basic enough for XFS, for instance. > They just have to error on the side of being cautious, by going full > sector alignment for memory... How does the filesystem find out about hardware alignment requirements? Isn't probing through the block device to find out about the request queue configurations considered a layering violation? What if sector alignment is not sufficient? And how would this work if we start supporting sector sizes larger than page size? (which the XFS buffer cache supports just fine, even if nothing else in Linux does). But even ignoring sector size > page size, implementing this requires a bunch of new slab caches, especially for 64k page machines because XFS supports sector sizes up to 32k. And every other filesystem that uses sector sized buffers (e.g. HFS) would have to do the same thing. Seems somewhat wasteful to require everyone to implement their own aligned sector slab cache... Perhaps we should take the filesystem out of this completely - maybe the block layer could provide a generic "sector heap" and have all filesystems that use sector sized buffers allocate from it. e.g. something like mem = bdev_alloc_sector_buffer(bdev, sector_size) That way we don't have to rely on filesystems knowing anything about the alignment limitations of the devices or assumptions about DMA to work correctly... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
