On Tue, 2016-06-21 at 14:17 -0400, Mike Snitzer wrote: > On Tue, Jun 21 2016 at 11:44am -0400, > Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote: > > > > On Tue, 2016-06-21 at 09:41 -0400, Mike Snitzer wrote: > > > > > > On Mon, Jun 20 2016 at 6:22pm -0400, > > > Mike Snitzer <snitzer@xxxxxxxxxx> wrote: : > > > I'm now wondering if we'd be better off setting a new QUEUE_FLAG_DAX > > > rather than establish GENHD_FL_DAX on the genhd? > > > > > > It'd be quite a bit easier to allow upper layers (e.g. XFS and ext4) to > > > check for a queue flag. > > > > I think GENHD_FL_DAX is more appropriate since DAX does not use a request > > queue, except for protecting the underlining device being disabled while > > direct_access() is called (b2e0d1625e19). > > The devices in question have a request_queue. All bio-based device have > a request_queue. DAX-capable devices have two operation modes, bio-based and DAX. I agree that bio-based operation is associated with a request queue, and its capabilities should be set to it. DAX, on the other hand, is rather independent from a request queue. > I don't have a big problem with GENHD_FL_DAX. Just wanted to point out > that such block device capabilities are generally advertised in terms of > a QUEUE_FLAG. I do not have a strong opinion, but feel a bit odd to associate DAX to a request queue. > > About protecting direct_access, this patch assumes that the underlining > > device cannot be disabled until dtr() is called. Is this correct? If > > not, I will need to call dax_map_atomic(). > > One of the big design considerations for DM that a DM device can be > suspended (with or without flush) and any new IO will be blocked until > the DM device is resumed. > > So ideally DM should be able to have the same capability even if using > DAX. Supporting suspend for DAX is challenging since it allows user applications to access a device directly. Once a device range is mmap'd, there is no kernel intervention to access the range, unless we invalidate user mappings. This isn't done today even after a driver is unbind'd from a device. > But that is different than what commit b2e0d1625e19 is addressing. For > DM, I wouldn't think you'd need the extra protections that > dax_map_atomic() is providing given that the underlying block device > lifetime is managed via DM core's dm_get_device/dm_put_device (see also: > dm.c:open_table_device/close_table_device). I thought so as well. But I realized that there is (almost) nothing that can prevent the unbind operation. It cannot fail, either. This unbind proceeds even when a device is in-use. In case of a pmem device, it is only protected by pmem_release_queue(), which is called when a pmem device is being deleted and calls blk_cleanup_queue() to serialize a critical section between blk_queue_enter() and blk_queue_exit() per b2e0d1625e19. This prevents from a kernel DTLB fault, but does not prevent a device disappeared while in-use. Protecting DM's underlining device with blk_queue_enter() (or something similar) requires more thoughts... blk_queue_enter() to a DM device cannot be redirected to its underlining device. So, this is TBD for now. But I do not think this is a blocker issue since doing unbind to a underlining device is quite harmful no matter what we do - even if it is protected with blk_queue_enter(). Thanks, -Toshi -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
