linux-kernel - Re: [PATCH 6/6] fs: xfs: Set FMODE_CAN_ATOMIC_WRITE for FS_XFLAG

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Date: Fri, 23 Feb 2024 20:18:30 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
	dchinner@...hat.com, jack@...e.cz, chandan.babu@...cle.com,
	martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH 6/6] fs: xfs: Set FMODE_CAN_ATOMIC_WRITE for
 FS_XFLAG_ATOMICWRITES set

On Wed, Feb 21, 2024 at 05:38:39PM +0000, John Garry wrote:
> 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.

<nod> "check in the bio submit path" sounds good to me.  Adding in
redundant checks which are eventually gated on whatever submit_bio does
sounds like excessive overhead and layering violations.

> > 
> > 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?

Right.  fsdax doesn't support atomic memcpy to pmem.

--D

> 
> Thanks,
> John
> 
>