linux-kernel - Re: [PATCH 5/6] iomap: Lift blocksize restriction on atomic writes

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Message-ID: <fc6fddee-2707-4cca-b0b7-983c8dd17e16@oracle.com>
Date: Fri, 25 Oct 2024 10:59:51 +0100
From: John Garry <john.g.garry@...cle.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>, linux-ext4@...r.kernel.org
Cc: Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Christoph Hellwig
 <hch@...radead.org>,
        Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/6] iomap: Lift blocksize restriction on atomic writes

On 25/10/2024 10:31, Ritesh Harjani (IBM) wrote:
>>>    
>>> -	if (atomic && length != fs_block_size)
>>> +	if (atomic && length != iter->len)
>>>    		return -EINVAL;
>> Here you expect just one iter for an atomic write always.
> Here we are lifting the limitation of iomap to support entire iter->len
> rather than just 1 fsblock.

Sure

> 
>> In 6/6, you are saying that iomap does not allow an atomic write which
>> covers unwritten and written extents, right?
> No, it's not that. If FS does not provide a full mapping to iomap in
> ->iomap_begin then the writes will get split. 

but why would it provide multiple mapping?

> For atomic writes this
> should not happen, hence the check in iomap above to return -EINVAL if
> mapped length does not match iter->len.
> 
>> But for writing a single fs block atomically, we don't mandate it to be
>> in unwritten state. So there is a difference in behavior in writing a
>> single FS block vs multiple FS blocks atomically.
> Same as mentioned above. We can't have atomic writes to get split.
> This patch is just lifting the restriction of iomap to allow more than
> blocksize but the mapped length should still meet iter->len, as
> otherwise the writes can get split.

Sure, I get this. But I wonder why would we be getting multiple 
mappings? Why cannot the FS always provide a single mapping?

Thanks,
John