[Added ext4, xfs, and linux-api folks to CC for the interface discussion]

On Tue 02-10-18 14:10:39, Johannes Thumshirn wrote:

On Tue, Oct 02, 2018 at 12:05:31PM +0200, Jan Kara wrote:

quoted

Hello,

commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and device dax" has
removed VM_MIXEDMAP flag from DAX VMAs. Now our testing shows that in the
mean time certain customer of ours started poking into /proc/<pid>/smaps
and looks at VMA flags there and if VM_MIXEDMAP is missing among the VMA
flags, the application just fails to start complaining that DAX support is
missing in the kernel. The question now is how do we go about this?

OK naive question from me, how do we want an application to be able to
check if it is running on a DAX mapping?

The question from me is: Should application really care? After all DAX is
just a caching decision. Sure it affects performance characteristics and
memory usage of the kernel but it is not a correctness issue (in particular
we took care for MAP_SYNC to return EOPNOTSUPP if the feature cannot be
supported for current mapping). And in the future the details of what we do
with DAX mapping can change - e.g. I could imagine we might decide to cache
writes in DRAM but do direct PMEM access on reads. And all this could be
auto-tuned based on media properties. And we don't want to tie our hands by
specifying too narrowly how the kernel is going to behave.

OTOH I understand that e.g. for a large database application the difference
between DAX and non-DAX mapping can be a difference between performs fine
and performs terribly / kills the machine so such application might want to
determine / force caching policy to save sysadmin from debugging why the
application is misbehaving.

AFAIU DAX is always associated with a file descriptor of some kind (be
it a real file with filesystem dax or the /dev/dax device file for
device dax). So could a new fcntl() be of any help here? IS_DAX() only
checks for the S_DAX flag in inode::i_flags, so this should be doable
for both fsdax and devdax.

So fcntl() to query DAX usage is one option. Another option is the GETFLAGS
ioctl with which you can query the state of S_DAX flag (works only for XFS
currently). But that inode flag was meant more as a hint "use DAX if
available" AFAIK so that's probably not really suitable for querying
whether DAX is really in use or not. Since DAX is really about caching
policy, I was also thinking that we could use madvise / fadvise for this.
I.e., something like MADV_DIRECT_ACCESS which would return with success if
DAX is in use, with error if not. Later, kernel could use it as a hint to
really force DAX on a mapping and not try clever caching policies...
Thoughts?

								Honza

-- 
Jan Kara [off-list ref]
SUSE Labs, CR