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

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Message-ID: <63587581-17a5-431e-9fe3-a1a24ea4fa21@oracle.com>
Date: Wed, 12 Mar 2025 09:04:07 +0000
From: John Garry <john.g.garry@...cle.com>
To: Christoph Hellwig <hch@...radead.org>
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 12/03/2025 07:39, Christoph Hellwig wrote:
> 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.

Then we would need to limit the awu max to whatever can be guaranteed
(to fit).


> 
>> +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?
> 

Yes, I suppose that it could always be used. I would suggest that as a 
later improvement, if you agree.

Thanks,
John