linux-kernel - Re: [RFC PATCH 02/17] block: simplify bio_add

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Message-ID: <20141223102221.GB13138@infradead.org>
Date:	Tue, 23 Dec 2014 02:22:21 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Dongsu Park <dongsu.park@...fitbricks.com>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kmo@...erainc.com>,
	Ming Lin <mlin@...ggr.net>, target-devel@...r.kernel.org,
	Boaz Harrosh <ooo@...ctrozaur.com>,
	Benny Halevy <bhalevy@...marydata.com>
Subject: Re: [RFC PATCH 02/17] block: simplify bio_add_page()

On Mon, Dec 22, 2014 at 12:48:29PM +0100, Dongsu Park wrote:
> From: Kent Overstreet <kmo@...erainc.com>
> 
> Since generic_make_request() can now handle arbitrary size bios, all we
> have to do is make sure the bvec array doesn't overflow.
> __bio_add_page() doesn't need to call ->merge_bvec_fn(), where
> we can get rid of unnecessary code paths.

This needs an explanation of why removign the call to ->merge_bvec_fn
is fine for bio_add_pc_page.  I guess it's because neither
the target pscsi pass through mode, nor the osd code ever use anything
but a simple scsi devices that doesn't even have one, but it needs to be
clearly spelled out.

> + *	Attempt to add a page to the bio_vec maplist. This will only fail if
> + *	bio->bi_vcnt == bio->bi_max_vecs.

It also fails on a cloned bio, although that might better be turned into
a BUG_ON().

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