On Mon, Jun 04, 2018 at 08:46:28PM -0400, Mike Snitzer wrote: > On Mon, Jun 04 2018 at 7:24pm -0400, > Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > > On Fri, Jun 01, 2018 at 06:04:43PM -0400, Mike Snitzer wrote: > > > On Tue, May 29 2018 at 3:51pm -0400, > > > Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > > > > > > The DM_TYPE_DAX_BIO_BASED dm_queue_mode was introduced to prevent DM > > > > devices that could possibly support DAX from transitioning into DM devices > > > > that cannot support DAX. > > > > > > > > For example, the following transition will currently fail: > > > > > > > > dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw] > > > > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED > > > > > > > > but these will both succeed: > > > > > > > > dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw] > > > > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED > > > > > > > > > > I fail to see how this succeeds given > > > drivers/md/dm-ioctl.c:is_valid_type() only allows transitions from: > > > > > > DM_TYPE_BIO_BASED => DM_TYPE_DAX_BIO_BASED > > > > Right, sorry, that was a typo. What I meant was: > > > > > For example, the following transition will currently fail: > > > > > > dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw] > > > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED > > > > > > but these will both succeed: > > > > > > dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw] > > > DM_TYPE_BIO_BASED DM_TYPE_BIO_BASED > > > > > > dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem] > > > DM_TYPE_BIO_BASED DM_TYPE_DAX_BIO_BASED > > > > So we allow 2 of the 3 transitions, but the reason that we disallow the third > > isn't fully clear to me. > > > > > > dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem] > > > > DM_TYPE_BIO_BASED DM_TYPE_DAX_BIO_BASED > > > > > > > > This seems arbitrary, as really the choice on whether to use DAX happens at > > > > filesystem mount time. There's no guarantee that the in the first case > > > > (double fsdax pmem) we were using the dax mount option with our file > > > > system. > > > > > > > > Instead, get rid of DM_TYPE_DAX_BIO_BASED and all the special casing around > > > > it, and instead make the request queue's QUEUE_FLAG_DAX be our one source > > > > of truth. If this is set, we can use DAX, and if not, not. We keep this > > > > up to date in table_load() as the table changes. As with regular block > > > > devices the filesystem will then know at mount time whether DAX is a > > > > supported mount option or not. > > > > > > If you don't think you need this specialization that is fine.. but DM > > > devices supporting suspending (as part of table reloads) so is there any > > > risk that there will be inflight IO (say if someone did 'dmsetup suspend > > > --noflush').. and then upon reload the device type changed out from > > > under us.. anyway, I don't have all the PMEM DAX stuff paged back into > > > my head yet. > > > > > > But this just seems like we really shouldn't be allowing the > > > transition from what was DM_TYPE_DAX_BIO_BASED back to DM_TYPE_BIO_BASED > > > > I admit I don't fully understand all the ways that DM supports suspending and > > resuming devices. Is there actually a case where we can change out the DM > > devices while I/O is running, and somehow end up trying to issue a DAX I/O to > > a device that doesn't support DAX? > > Yes, provided root permissions, it's very easy to dmsetup suspend/load/resume > to replace any portion of the DM device's logical address space to map to an > entirely different DM target (with a different backing store). It's > pretty intrusive to do such things, but easily done and powerful. > > Mike Hmmm, I don't understand how you can do this if there is a filesystem built on your DM device? Say you have a DM device, either striped or linear, that is made up of 2 devices, and then you use dmsetup to replace one of the DM member devices with something else. You've just swapped out half of your LBA space with new data, right? I don't understand how you can expect a filesystem built on the old DM device to still work? You especially can't do this while the filesystem is mounted - all the in-core filesystem metadata would be garbage because the on-media data would have totally changed. So, when dealing with a filesystem, the flow must be: unmount your filesystem redo your DM device, changing out devices reformat your filesystem on the new DM device remount your filesystem Right? If so, then I don't see how a transition of the DM device from supporting DAX to not supporting DAX or vice versa could harm us, as we can't be doing filesystem I/O at the time when we change the composition of the DM device.
