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Date:   Wed, 8 Jan 2020 05:37:35 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Carlos Maiolino <cmaiolino@redhat.com>,
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Subject: Re: [PATCH V2] block: add bio_truncate to fix guard_bio_eod
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> +void bio_truncate(struct bio *bio, unsigned new_size)

This function really needs a kerneldoc or similar comment describing
what it does in detail.

> +	if (bio_data_dir(bio) != READ)
> +		goto exit;

This really should check the passed in op for REQ_OP_READ directly instead
of just the direction on the potentially not fully set up bio.