On Wed, Jun 22, 2016 at 10:44 AM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote: > 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. Given that we do not support dax to a raw block device [1] it seems a gendisk flag is more misleading than request_queue flag that specifies what requests can be made of the device. [1]: acc93d30d7d4 Revert "block: enable dax for raw block devices" >> > 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(). I still have the "block device removed" notification patches on my todo list. It's not a blocker, but there are scenarios where we can keep accessing memory via dax of a disabled device leading to memory corruption. I'll bump that up in my queue now that we are looking at additional scenarios where letting DAX mappings leak past the reconfiguration of a block device could lead to trouble. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
