linux-kernel - Re: [PATCH v7 11/14] xfs: add xfs_file_dio_write

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Message-ID: <69302bf1-78b4-4b95-8e9b-df56dd1091c0@oracle.com>
Date: Tue, 22 Apr 2025 07:08:32 +0100
From: John Garry <john.g.garry@...cle.com>
To: Luis Chamberlain <mcgrof@...nel.org>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.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,
        Pankaj Raghav <p.raghav@...sung.com>,
        Daniel Gomez <da.gomez@...sung.com>
Subject: Re: [PATCH v7 11/14] xfs: add xfs_file_dio_write_atomic()

On 21/04/2025 22:18, Luis Chamberlain wrote:
>> /*
>> +	 * The retry mechanism is based on the ->iomap_begin method returning
>> +	 * -ENOPROTOOPT, which would be when the REQ_ATOMIC-based write is not
>> +	 * possible. The REQ_ATOMIC-based method typically not be possible if
>> +	 * the write spans multiple extents or the disk blocks are misaligned.
>> +	 */
>> +	if (ret == -ENOPROTOOPT && dops == &xfs_direct_write_iomap_ops) {
> Based on feedback from LSFMM, due to the performance variaibility this
> can introduce, it sounded like some folks would like to opt-in to not
> have a software fallback and just require an error out.
 > > Could an option be added to not allow the software fallback?

I still don't see the use in this.

So consider userspace wants to write something atomically and we fail as 
a HW-based atomic write is not possible. What is userspace going to do next?

I heard something like "if HW-based atomics are not possible, then 
something has not been configured properly for the FS" - that something 
would be extent granularity and alignment, but we don't have a method to 
ensure this. That is the whole point of having a FS fallback.

> 
> If so, then I think the next patch would also need updating.
> 
> Or are you suggesting that without the software fallback atomic writes
> greater than fs block size are not possible?

Yes, as XFS has no method to guarantee extent granularity and alignment.