On Fri, Jun 01, 2018 at 04:46:04PM -0400, Mike Snitzer wrote: > On Fri, Jun 01 2018 at 4:19P -0400, > Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > > > On Tue, May 29 2018 at 3:51P -0400, > > Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > > > > Currently device_supports_dax() just checks to see if the QUEUE_FLAG_DAX > > > flag is set on the device's request queue to decide whether or not the > > > device supports filesystem DAX. This is insufficient because there are > > > devices like PMEM namespaces in raw mode which have QUEUE_FLAG_DAX set but > > > which don't actually support DAX. > > > > Isn't that a PMEM bug then? > > > > What is the point of setting QUEUE_FLAG_DAX if it cannot be trusted? Yup, that seems like a bug. > > > This means that you could create a dm-linear device, for example, where the > > > first part of the dm-linear device was a PMEM namespace in fsdax mode and > > > the second part was a PMEM namespace in raw mode. Both DM and the > > > filesystem you put on that dm-linear device would think the whole device > > > supports DAX, which would lead to bad behavior once your raw PMEM namespace > > > part using DAX needed struct page for something. > > > > The PMEM namespace in raw mode shouldn't be setting QUEUE_FLAG_DAX, if > > it didn't then the stacked-up linear DM wouldn't > > > > > Fix this by using bdev_dax_supported() like filesystems do at mount time. > > > This checks for raw mode and also performs other tests like checking to > > > make sure the dax_direct_access() path works. > > > > Sorry "This" does those things where? > > I see you meant bdev_dax_supported() does these additional checks. > > My previous question stands though. Why is QUEUE_FLAG_DAX getting set > if the device hasn't already passed these checks? Shouldn't setting > QUEUE_FLAG_DAX on request_queue depend on bdev_dax_supported() passing? > > But looking at the drivers that do set QUEUE_FLAG_DAX: they > don't have the bdev readily available. Anyway, just strikes me as > bizarre that a driver can set QUEUE_FLAG_DAX without having to have > ensured bdev_dax_supported() passes (even if not programatically, but > that the developer has verified the hooks, et al exist). I agree that it's a bug that QUEUE_FLAG_DAX is set for raw mode namespaces, thanks for pointing that out. I will fix that, but I still think that DM should be using bdev_dax_supported() like the filesystems do. Even beyond the raw mode / fsdax mode thing, this does additional checks that are useful. So, I think these DM patches are still correct. > But I'll give up on this line of questioning.. > > My dilemma now is: how do I take these changes without first rebasing > linux-dm.git ontop of Darrick's xfs tree? > > I probably should've reviewed faster and been the one to take the entire > set (with appropriate acks obviously). -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
