linux-kernel - Re: [PATCH v6 13/13] block: Only clone bio vecs that are in use

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Message-ID: <20120822211045.GN19212@google.com>
Date:	Wed, 22 Aug 2012 14:10:45 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, vgoyal@...hat.com, mpatocka@...hat.com,
	bharrosh@...asas.com, Jens Axboe <axboe@...nel.dk>,
	Alasdair Kergon <agk@...hat.com>, Sage Weil <sage@...tank.com>
Subject: Re: [PATCH v6 13/13] block: Only clone bio vecs that are in use

Hello, Kent.

On Wed, Aug 22, 2012 at 10:04:10AM -0700, Kent Overstreet wrote:
> bcache creates large bios internally, and then splits them according to
> the device requirements before it sends them down. If a lower level
> device tries to clone the bio, and the original bio had more than
> BIO_MAX_PAGES, the clone will fail unecessarily.
> 
> We can fix this by only cloning the bio vecs that are actually in use.

I'm pretty sure I sound like a broken record by now, but

* How was this tested?

* What are the implications and possible dangers?

> @@ -463,10 +468,10 @@ void __bio_clone(struct bio *bio, struct bio *bio_src)
>  	bio->bi_sector = bio_src->bi_sector;
>  	bio->bi_bdev = bio_src->bi_bdev;
>  	bio->bi_flags |= 1 << BIO_CLONED;
> +	bio->bi_flags &= ~(1 << BIO_SEG_VALID);

For the n'th time, explain please.

Thanks.

-- 
tejun
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