Re: [PATCH 04/18] dax: Introduce IOMAP_DAX_COW to CoW edges during writes

From: Shiyang Ruan <ruansy.fnst@cn.fujitsu.com>
To: Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc: kilobyte@angband.pl, jack@suse.cz,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	nborisov@suse.com, linux-nvdimm@lists.01.org,
	david@fromorbit.com, dsterba@suse.cz, willy@infradead.org,
	linux-fsdevel@vger.kernel.org, hch@lst.de,
	linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 04/18] dax: Introduce IOMAP_DAX_COW to CoW edges during writes
Date: Mon, 27 May 2019 16:25:41 +0800	[thread overview]
Message-ID: <1620c513-4ce2-84b0-33dc-2675246183ea@cn.fujitsu.com> (raw)
In-Reply-To: <20190523115109.2o4txdjq2ft7fzzc@fiona>

On 5/23/19 7:51 PM, Goldwyn Rodrigues wrote:
>>
>> Hi,
>>
>> I'm working on reflink & dax in XFS, here are some thoughts on this:
>>
>> As mentioned above: the second iomap's offset and length must match the
>> first.  I thought so at the beginning, but later found that the only
>> difference between these two iomaps is @addr.  So, what about adding a
>> @saddr, which means the source address of COW extent, into the struct iomap.
>> The ->iomap_begin() fills @saddr if the extent is COW, and 0 if not.  Then
>> handle this @saddr in each ->actor().  No more modifications in other
>> functions.
> 
> Yes, I started of with the exact idea before being recommended this by Dave.
> I used two fields instead of one namely cow_pos and cow_addr which defined
> the source details. I had put it as a iomap flag as opposed to a type
> which of course did not appeal well.
> 
> We may want to use iomaps for cases where two inodes are involved.
> An example of the other scenario where offset may be different is file
> comparison for dedup: vfs_dedup_file_range_compare(). However, it would
> need two inodes in iomap as well.
> 
Yes, it is reasonable.  Thanks for your explanation.

One more thing RFC:
I'd like to add an end-io callback argument in ->dax_iomap_actor() to 
update the metadata after one whole COW operation is completed.  The 
end-io can also be called in ->iomap_end().  But one COW operation may 
call ->iomap_apply() many times, and so does the end-io.  Thus, I think 
it would be nice to move it to the bottom of ->dax_iomap_actor(), called 
just once in each COW operation.

-- 
Thanks,
Shiyang Ruan.
>>
>> My RFC patchset[1] is implemented in this way and works for me, though it is
>> far away from perfectness.
>>
>> [1]: https://patchwork.kernel.org/cover/10904307/
>>
> 

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