On Wed, Oct 03, 2018 at 06:44:07PM +0200, Jan Kara wrote:

On Wed 03-10-18 08:13:37, Dan Williams wrote:

quoted

On Wed, Oct 3, 2018 at 8:07 AM Jan Kara [off-list ref] wrote:

quoted

WRT per-inode DAX property, AFAIU that inode flag is just going to be
advisory thing - i.e., use DAX if possible. If you mount a filesystem with
these inode flags set in a configuration which does not allow DAX to be
used, you will still be able to access such inodes but the access will use
page cache instead. And querying these flags should better show real
on-disk status and not just whether DAX is used as that would result in an
even bigger mess. So this feature seems to be somewhat orthogonal to the
API I'm looking for.

True, I imagine once we have that flag we will be able to distinguish
the "saved" property and the "effective / live" property of DAX...
Also it's really not DAX that applications care about as much as "is
there page-cache indirection / overhead for this mapping?". That seems
to be a narrower guarantee that we can make than what "DAX" might
imply.

Right. So what do people think about my suggestion earlier in the thread to
use madvise(MADV_DIRECT_ACCESS) for this? Currently it would return success
when DAX is in use, failure otherwise. Later we could extend it to be also
used as a hint for caching policy for the inode...

Hmm apart from Dan's objection that it can't really be used for a
query, isn't madvise(2) for mmap(2)?

But AFAIU (from looking at the xfs code, so please correct me if I',
wrong), DAX can be used for the traditional read(2)/write(2) interface
as well.

There is at least:

xfs_file_read_iter()
`-> if (IS_DAX(inode))
    `-> xfs_file_dax_read()
        `->dax_iomap_rw()

So IMHO something on an inode granularity would make more sens to me.

Byte,
	Johannes
-- 
Johannes Thumshirn                                          Storage
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