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Date:   Wed, 18 May 2022 14:48:15 +0200
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>,
        David Sterba <dsterba@suse.com>, Qu Wenruo <wqu@suse.com>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 12/15] btrfs: add new read repair infrastructure
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On Wed, May 18, 2022 at 06:20:53PM +0800, Qu Wenruo wrote:
> My bad, I see the bio_alloc_bioset() but didn't check it's allocating a
> bi_io_vec with size 0, and soon utilize the original bi_io_vec.
>
> So the function matches its name, it's really bio clone.
>
> And it's very different from my version, which really allocates a new
> bio with larger enough bi_io_vec then adding back the needed sectors
> from the original bio.
>
> Then I guess the BIO_CLONE flag is completely fine.
>
> But in that case, you may want to call bio_alloc_clone() directly? Which
> can handle bioset without problem.

The big difference is that bio_alloc_clone directly looks at
bio->bi_iter, while we must look at btrfs_bio->iter as bio->bi_iter
may already be consumed by the driver.  I though the comment in the
code explains that, but maybe I need to improve it.