On Wed, Sep 6, 2017 at 10:07 AM, Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > On Tue, Sep 05, 2017 at 09:12:35PM -0500, Eric Sandeen wrote: >> On 9/5/17 5:35 PM, Ross Zwisler wrote: >> > The original intent of this series was to add a per-inode DAX flag to ext4 >> > so that it would be consistent with XFS. In my travels I found and fixed >> > several related issues in both ext4 and XFS. >> >> Hi Ross - >> >> hch had a lot of reasons to nuke the dax flag from orbit, and we just >> /disabled/ it in xfs due to its habit of crashing the kernel... > > Ah, sorry, I wasn't CC'd on those threads and missed them. For any interested > bystanders: > > https://www.spinics.net/lists/linux-ext4/msg57840.html > https://www.spinics.net/lists/linux-xfs/msg09831.html > https://www.spinics.net/lists/linux-xfs/msg10124.html > >> so a couple questions: >> >> 1) does this series pass hch's "test the per-inode DAX flag" fstest? > > Nope, it has the exact same problems as the XFS per-inode DAX flag. > >> 2) do we have an agreement that we need this flag at all, or is this >> just a parity item because xfs has^whad a per-inode flag? > > It was for parity, and because it allows admins finer grained control over > their system. Basically all things discussed in response to Lukas's original > patch in the first link above. I think it's more than parity. When pmem is slower than page cache it is actively harmful to have DAX enabled globally for a filesystem. So, not only should we push for per-inode DAX control, we should also push to deprecate the mount option. I agree with Christoph that we should try to automatically and transparently enable DAX where it makes sense, but we also need a finer-grained mechanism than a mount flag to force the behavior one way or the other.
