Re: [PATCH v8 15/15] xfs: allow sysadmins to specify a maximum atomic write limit at mount time

[Christoph Hellwig <hch@lst.de>]

On Tue, Apr 22, 2025 at 12:27:39PM +0000, John Garry wrote:
> From: "Darrick J. Wong" <djwong@kernel.org>
> 
> Introduce a mount option to allow sysadmins to specify the maximum size
> of an atomic write.  If the filesystem can work with the supplied value,
> that becomes the new guaranteed maximum.
> 
> The value mustn't be too big for the existing filesystem geometry (max
> write size, max AG/rtgroup size).  We dynamically recompute the
> tr_atomic_write transaction reservation based on the given block size,
> check that the current log size isn't less than the new minimum log size
> constraints, and set a new maximum.
> 
> The actual software atomic write max is still computed based off of
> tr_atomic_ioend the same way it has for the past few commits.

The cap is a good idea, but a mount option for something that has
strong effects for persistent application formats is a little suboptimal.
But adding a sb field and an incompat bit wouldn't be great either.

Maybe this another use case for a trusted xattr on the root inode like
the autofsck flag?