Re: [PATCH 6/6] fs: xfs: Set FMODE_CAN_ATOMIC_WRITE for FS_XFLAG_ATOMICWRITES set

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On 21/02/2024 17:00, Darrick J. Wong wrote:
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.

If that is the case then a check in the bio submit path is required to catch any such reconfigure problems - and we effectively have that in this series.

I am looking at change some of these XFS bdev_can_atomic_write() checks, but would still have a check in the bio submit path.


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.

Yes, so proper to use target->bt_bdev for checks for bdev atomic write capability, right?

Thanks,
John





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