linux-kernel - Re: [PATCH 01/27] block: bio: introduce 4 helpers for cleanup

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Date:	Tue, 5 Apr 2016 17:46:36 -0800
From:	Kent Overstreet <kent.overstreet@...il.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Jens Axboe <axboe@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
	Boaz Harrosh <boaz@...xistor.com>, Jan Kara <jack@...e.cz>,
	Keith Busch <keith.busch@...el.com>, Tejun Heo <tj@...nel.org>,
	Mike Snitzer <snitzer@...hat.com>
Subject: Re: [PATCH 01/27] block: bio: introduce 4 helpers for cleanup

On Wed, Apr 06, 2016 at 09:34:34AM +0800, Ming Lei wrote:
> On Wed, Apr 6, 2016 at 8:18 AM, Kent Overstreet
> <kent.overstreet@...il.com> wrote:
> > On Tue, Apr 05, 2016 at 07:56:46PM +0800, Ming Lei wrote:
> >> Some drivers access bio->bi_vcnt and bio->bi_io_vec directly,
> >> firstly it isn't a good practice, secondly it may cause trouble
> >> for converting to multipage bvecs.
> >
> > "not good practice" is OO bullshit snake oil without more justification. We
> > don't plaster accessors everywhere without an actual reason.
> >
> > How would it cause trouble with multipage bvecs?
> 
> Simply speaking, the current drivers may depend on .bi_vcnt for
> computing how many page there are in one bio. After multipage bvecs,
> it is not true any more. Isn't it a actual reason?

But it's completely valid to use bi_vcnt for segments, which is what it's always
_really_ meant anyways.

Sometimes you have cases where the meaning of a member changes significantly
enough that you really don't want code using it accidentally anymore - like with
Jens' patches that changed how bi_remaining and bi_cnt work, but after those
patches it really wasn't correct to use those members directly anymore so he
renamed them to prevent that.

I don't buy that that's the case for multipage bvecs - the meaning of bi_vcnt
itself isn't changing (it's just the number of entries in the array!) and it'll
still be possible for code to correctly use it directly.

Same with bio->bi_io_vec, it's still an array of biovecs, that's not changing.
Your helpers are at the wrong level of abstraction.

Also, there isn't a huge number of bi_vcnt references in the kernel anyways -
the immutable biovec work required removing most of them.

Instead of adding these low level accessors, it'd be better to convert code to
higher level helpers (especially bio_add_page()) where applicable.