Re: [RFC 1/2] block: allow other bd i_node flags when DAX is disabled

[Jon Derrick <jonathan.derrick@intel.com>]

On Fri, May 13, 2016 at 10:53:37AM -0700, Dan Williams wrote:
> On Fri, May 13, 2016 at 10:33 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> > Dan Williams <dan.j.williams@intel.com> writes:
> >
> >> On Thu, May 12, 2016 at 10:43 AM, Jon Derrick
> >> <jonathan.derrick@intel.com> wrote:
> >>> When DAX is not compiled into the kernel or the device does not support
> >>> direct-access, the block device file's inode flags are fully cleared.
> >>> This patch changes it to only clear the S_DAX flag when DAX is disabled.
> >>>
> >>> This reverts to i_flags behavior prior to
> >>> bbab37ddc20bae4709bca8745c128c4f46fe63c5
> >>>
> >>> Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> >>> ---
> >>>  fs/block_dev.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/fs/block_dev.c b/fs/block_dev.c
> >>> index 20a2c02..d4fa725 100644
> >>> --- a/fs/block_dev.c
> >>> +++ b/fs/block_dev.c
> >>> @@ -1208,7 +1208,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >>>                 if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops->direct_access)
> >>>                         bdev->bd_inode->i_flags = S_DAX;
> >>>                 else
> >>> -                       bdev->bd_inode->i_flags = 0;
> >>> +                       bdev->bd_inode->i_flags &= ~S_DAX;
> >>
> >> Setting S_DAX is atomic, but the above change makes it a non-atomic
> >> read-modify-write.  Do we need exclusion / locking in this path?
> >
> > First, I don't see how you'd ever get a read-modify-write here, as S_DAX
> > surely won't ever be set if direct_access isn't supported.  Second,
> > there is existing code that already does this very thing.  See further
> > down in __blkdev_get:
> >
> >                         if (!ret) {
> >                                 bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
> >                                 if (!blkdev_dax_capable(bdev))
> >                                         bdev->bd_inode->i_flags &= ~S_DAX;
> >                         }
> >
> > What relies on S_DAX being set or cleared atomically?
> 
> So, when I wrote the above "&= ~S_DAX" I was worried about the
> non-atomic update with respect to the BLKDAXSET ioctl, but we killed
> BLKDAXSET so the worry went away.  Now an inode flag setting ioctl is
> back for the S_HIPRI case.  Can that collide with these other flag
> touches?

The S_DAX case is unclear to me, but the new ioctl certainly does need a i_mutex lock and to use inode_set_flags.