On Thu, Sep 14, 2017 at 02:57:41PM +0800, Eryu Guan wrote: > Hi Ross, > > On Mon, Sep 11, 2017 at 02:01:03PM -0600, Ross Zwisler wrote: > > This adds a regression test for the following kernel patch: > > > > xfs: always use DAX if mount option is used > > > > This test will also pass with kernel v4.14-rc1 and beyond because the XFS > > DAX I/O mount option has been disabled (but not removed), so the > > "chattr -x" to turn off DAX doesn't actually do anything. > > > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > > Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx> > > --- > > > > Changes since v1: > > - Use perf instead of tracepoints to detect whether DAX is used. (Dan) > > Thanks for the test! But I agreed with Dave here, it doesn't seem like a > good idea to depend on the kernel tracepoints in a test, but I can't > think of a better solution either, so I didn't get to this patch > earlier.. > > Before XFS disabled the ability to switch on & off per-inode DAX flag, > the x flag was only shown after an explicit 'chattr +x', even if XFS was > mounted with dax option, e.g. > > # mkfs -t xfs -f /dev/ram0 > # mount -o dax /dev/ram0 /mnt/xfs > # echo "test" > /mnt/xfs/testfile > # xfs_io -c "lsattr" /mnt/xfs/testfile > ---------------- /mnt/xfs/testfile > # xfs_io -c "chattr +x" /mnt/xfs/testfile > # xfs_io -c "lsattr" /mnt/xfs/testfile > ---------------x /mnt/xfs/testfile XFS actually still works this way, you just don't get dax now when you chattr +x. :-/ But the inode flag is actually still there, gets updated by chattr and can be listed with lsattr. Actually, that feels like a really bad situation to be in - Christoph & Dave, should we do more to remove the flag as long as it's not working? i.e. remove it from the lsattr output and make "chattr +x" fail with -EINVAL or similar? > I'm wondering if it makes sense to make lsattr print the x flag by > default when XFS is mounted with dax option, that way, we have a method > to know whether dax is used or not on a particular file too. Well, the per-inode flag that gets set in the filesystem metadata and the actual S_DAX runtime flag which controls whether or not we do DAX I/O and page faults are different things, and as we've seen they aren't always synchronized. I think making the 'x' flag in lsattr reflect the current state of S_DAX is interesting, but it would suffer from the same TOCTOU races that Dave was concerned about for the proposed VM_DAX flag: https://lkml.org/lkml/2016/9/16/13 It could also be surprising to users - if they had mounted with -o dax, lsattr on each of their files would show the 'x' flag, but if they remount without that option those 'x' flags would go away. I think this is surprising because normally it takes a chattr to modify the flags. -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
