linux-kernel - Re: [dm-devel] [PATCH v3 09/26] block: Remove bi

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Message-ID: <20121002201620.GM26488@google.com>
Date:	Tue, 2 Oct 2012 13:16:20 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, tj@...nel.org, axboe@...nel.dk
Subject: Re: [dm-devel] [PATCH v3 09/26] block: Remove bi_idx references

On Tue, Oct 02, 2012 at 03:24:34PM -0400, Vivek Goyal wrote:
> On Mon, Sep 24, 2012 at 03:34:49PM -0700, Kent Overstreet wrote:
> 
> [..]
> > diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
> > index 95e52879..24e5cef 100644
> > --- a/drivers/block/floppy.c
> > +++ b/drivers/block/floppy.c
> > @@ -3778,7 +3778,6 @@ static int __floppy_read_block_0(struct block_device *bdev)
> >  	bio_vec.bv_len = size;
> >  	bio_vec.bv_offset = 0;
> >  	bio.bi_vcnt = 1;
> > -	bio.bi_idx = 0;
> 
> So leaving bio.bi_idx uninitialized is safer than setting it to zero? Can
> you explain a bit more that why this change is required? 

It's not uninitialized, because it just called bio_init() - it was
entirely redundant before.

It's not strictly necessary, but immutable bio vecs require getting rid
of a lot of bi_idx usage, and everything that's left is either
unecessary or better/cleaner done some other way, so it's best to just
get rid of all of them - immutable bvecs changes the semantics of bi_idx
just enough that directly using it tends to be wrong.
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