Re: [PATCH v2 3/7] dm: fix test for DAX device support

[Ross Zwisler <ross.zwisler@linux.intel.com>]

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@redhat.com> wrote:
> 
> > On Tue, May 29 2018 at  3:51P -0400,
> > Ross Zwisler <ross.zwisler@linux.intel.com> 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).