linux-kernel - Re: [PATCH v3 14/21] iomap: Sub-extent zeroing

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Message-ID: <f740c31e-609a-443f-95d3-56fc92bb5d48@oracle.com>
Date: Mon, 29 Jul 2024 18:02:38 +0100
From: John Garry <john.g.garry@...cle.com>
To: Christoph Hellwig <hch@....de>
Cc: Dave Chinner <david@...morbit.com>, djwong@...nel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH v3 14/21] iomap: Sub-extent zeroing

On 26/07/2024 18:13, Christoph Hellwig wrote:
> On Fri, Jul 26, 2024 at 03:29:48PM +0100, John Garry wrote:
>> I have been considering another approach to solve this problem.
>>
>> In this patch - as you know - we zero unwritten parts of a newly allocated
>> extent. This is so that when we later issue an atomic write, we would not
>> have the problem of unwritten extents and how the iomap iterator will
>> create multiple BIOs (which is not permitted).
>>
>> How about an alternate approach like this:
>> - no sub-extent zeroing
>> - iomap iter is changed to allocate a single BIO for an atomic write in
>> first iteration
>> - each iomap extent iteration appends data to that same BIO
>> - when finished iterating, we submit the BIO
>>
>> Obviously that will mean many changes to the iomap bio iterator, but is
>> quite self-contained.
> 
> Yes, I also suggested that during the zeroing fix discussion. 

Maybe missed that. I did notice 
https://lore.kernel.org/linux-xfs/ZmwJuiMHQ8qgkJDS@infradead.org, but 
got a different impression of your idea there (to this one).

> There
> is generally no good reason to start a new direct I/O bio if the
> write is contiguous on disk and only the state of the srcmap is different.

Sure, so we don't need to worry about partially-completed writes, if 
that was a concern; and it would also mean dropping that unpleasant code 
change in xfs_iomap_write_unwritten() where the start/count fsb were 
being rounded out to the extent granule boundary.

> This will also be a big win for COW / out of place overwrites.
>