On Wed, Feb 14, 2024 at 12:36:40PM +0000, John Garry wrote:
> On 13/02/2024 17:59, Darrick J. Wong wrote:
> > > > Shouldn't we check that the device supports AWU at all before turning on
> > > > the FMODE flag?
> > > Can we easily get this sort of bdev info here?
> > >
> > > Currently if we do try to issue an atomic write and AWU for the bdev is
> > > zero, then XFS iomap code will reject it.
> > Hmm. Well, if we move towards pushing all the hardware checks out of
> > xfs/iomap and into whatever goes on underneath submit_bio then I guess
> > we don't need to check device support here at all.
>
> Yeah, I have been thinking about this. But I was still planning on putting a
> "bdev on atomic write" check here, as you mentioned.
>
> But is this a proper method to access the bdev for an xfs inode:
>
> STATIC bool
> xfs_file_can_atomic_write(
> struct xfs_inode *inode)
> {
> struct xfs_buftarg *target = xfs_inode_buftarg(inode);
> struct block_device *bdev = target->bt_bdev;
>
> if (!xfs_inode_atomicwrites(inode))
> return false;
>
> return bdev_can_atomic_write(bdev);
> }
There's still a TOCTOU race problem if the bdev gets reconfigured
between xfs_file_can_atomic_write and submit_bio.
However, if you're only using this to advertise the capability via statx
then I suppose that's fine -- userspace has to have some means of
discovering the ability at all. Userspace is also inherently racy.
> I do notice the dax check in xfs_bmbt_to_iomap() when assigning iomap->bdev,
> which is creating some doubt?
Do you mean this?
if (mapping_flags & IOMAP_DAX)
iomap->dax_dev = target->bt_daxdev;
else
iomap->bdev = target->bt_bdev;
The dax path wants dax_dev set so that it can do the glorified memcpy
operation, and it doesn't need (or want) a block device.
--D
> Thanks,
> John
>
