Messages in this thread | | | Date | Fri, 16 Sep 2016 11:24:59 +1000 | From | Dave Chinner <> | Subject | Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs |
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On Thu, Sep 15, 2016 at 05:16:42PM -0700, Dan Williams wrote: > On Thu, Sep 15, 2016 at 4:07 PM, Dave Chinner <david@fromorbit.com> wrote: > > On Thu, Sep 15, 2016 at 10:01:03AM -0700, Dan Williams wrote: > >> On Thu, Sep 15, 2016 at 1:26 AM, Christoph Hellwig <hch@lst.de> wrote: > >> > On Wed, Sep 14, 2016 at 11:54:38PM -0700, Dan Williams wrote: > >> >> The DAX property, page cache bypass, of a VMA is only detectable via the > >> >> vma_is_dax() helper to check the S_DAX inode flag. However, this is > >> >> only available internal to the kernel and is a property that userspace > >> >> applications would like to interrogate. > >> > > >> > They have absolutely no business knowing such an implementation detail. > >> > >> Hasn't that train already left the station with FS_XFLAG_DAX? > > > > No, that's an admin flag, not a runtime hint for applications. Just > > because that flag is set on an inode, it does not mean that DAX is > > actually in use - it will be ignored if the backing dev is not dax > > capable. > > What's the point of an admin flag if an admin can't do cat /proc/<pid > of interest>/smaps, or some other mechanism, to validate that the > setting the admin cares about is in effect?
Sorry, I don't follow - why would you be looking at mapping file regions in /proc to determine if some file somewhere in a filesystem has a specific flag set on it or not?
FS_XFLAG_DAX is an inode attribute flag, not something you can query or administrate through mmap:
I.e. # xfs_io -c "lsattr" -c "chattr +x" -c lsattr -c "chattr -x" -c "lsattr" foo --------------- foo --------------x foo --------------- foo #
What happens when that flag is set on an inode is determined by a whole bunch of other things that are completely separate to the management of the inode flag itself.
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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