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? Toshi, do you have a test case that shows this somehow?
