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

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Message-ID: <20250423083209.GA30432@lst.de>
Date: Wed, 23 Apr 2025 10:32:09 +0200
From: Christoph Hellwig <hch@....de>
To: John Garry <john.g.garry@...cle.com>
Cc: brauner@...nel.org, djwong@...nel.org, hch@....de,
	viro@...iv.linux.org.uk, jack@...e.cz, cem@...nel.org,
	linux-fsdevel@...r.kernel.org, dchinner@...hat.com,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	ojaswin@...ux.ibm.com, ritesh.list@...il.com,
	martin.petersen@...cle.com, linux-ext4@...r.kernel.org,
	linux-block@...r.kernel.org, catherine.hoang@...cle.com,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v8 15/15] xfs: allow sysadmins to specify a maximum
 atomic write limit at mount time

On Tue, Apr 22, 2025 at 12:27:39PM +0000, John Garry wrote:
> From: "Darrick J. Wong" <djwong@...nel.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?