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From:   Christoph Hellwig <hch@lst.de>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Christoph Hellwig <hch@lst.de>, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
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        Naohiro Aota <naohiro.aota@wdc.com>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        Qu Wenruo <wqu@suse.com>, Jens Axboe <axboe@kernel.dk>,
        "Darrick J. Wong" <djwong@kernel.org>, linux-block@vger.kernel.org,
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Subject: Re: [PATCH 04/17] btrfs: handle checksum validation and repair at
 the storage layer
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On Thu, Sep 01, 2022 at 05:04:34PM +0800, Qu Wenruo wrote:
> But for the verification part, I still don't like the idea of putting
> the verification code at endio context at all.

Why?

> This is especially true when data and metadata are still doing different
> checksum verfication at different timing.

Note that this does not handle the metadata checksum verification at
all.  Both because it actually works very different and I could not
verify that we'd actually always read all data that needs to be verified
together for metadata, but also because there is zero metadata repair
coverage in xfstests, so I don't dare to touch that code.

> Can we just let the endio function to do the IO, and let the reader to
> do the verification after all needed data is read out?

What would the benefit be?  It will lead to a lot of duplicate (and thus
inconsistent) code that is removed here, and make splitting the bios
under btrfs_submit_bio much more complicated and expensive.