linux-kernel - Re: [PATCH v5 07/10] xfs: Commit CoW-based atomic writes atomically

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Message-ID: <Z9E6LmV1PHOoEME7@infradead.org>
Date: Wed, 12 Mar 2025 00:39:26 -0700
From: Christoph Hellwig <hch@...radead.org>
To: John Garry <john.g.garry@...cle.com>
Cc: brauner@...nel.org, djwong@...nel.org, cem@...nel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, ojaswin@...ux.ibm.com,
	ritesh.list@...il.com, martin.petersen@...cle.com
Subject: Re: [PATCH v5 07/10] xfs: Commit CoW-based atomic writes atomically

On Mon, Mar 10, 2025 at 06:39:43PM +0000, John Garry wrote:
> When completing a CoW-based write, each extent range mapping update is
> covered by a separate transaction.
> 
> For a CoW-based atomic write, all mappings must be changed at once, so
> change to use a single transaction.

As already mentioned in a previous reply:  "all" might be to much.
The code can only support a (relatively low) number of extents
in a single transaction safely.

> +int
> +xfs_reflink_end_atomic_cow(
> +	struct xfs_inode		*ip,
> +	xfs_off_t			offset,
> +	xfs_off_t			count)

Assuming we could actually to the multi extent per transaction
commit safely, what would be the reason to not always do it?