Re: [PATCH] block: Make __bio_clone_fast() copy bi_vcnt

[Kent Overstreet <kent.overstreet@gmail.com>]

On Fri, Jun 29, 2018 at 07:10:47AM +0800, Ming Lei wrote:
> On Thu, Jun 28, 2018 at 11:21 PM, Bart Van Assche
> <bart.vanassche@wdc.com> wrote:
> > On 06/27/18 17:30, Ming Lei wrote:
> >>
> >> One core idea of immutable bvec is to use bio->bi_iter and the original
> >> bvec table to iterate over anywhere in the bio. That is why .bi_io_vec
> >> needs to copy, but not see any reason why .bi_vcnt needs to do.
> >>
> >> Do you have use cases on .bi_vcnt for cloned bio?
> >
> >
> > So far this is only a theoretical concern. There are many functions in the
> > block layer that use .bi_vcnt, and it is a lot of work to figure out all the
> > callers of all these functions.

Back when I implemented immutable biovecs I thoroughly audited all the bi_vcnt
uses and removed all of them that weren't by the code that owns/submits the bio.

Grepping around I see one or two suspicious uses.. blk-merge.c in particular

> No, any functions using .bi_vcnt on a cloned-bio may be a bug, and we should
> take a close look.

not just cloned bios, any code using bi_vcnt on a bio it didn't create is wrong.

so big nack to this patch (I wasn't ccd on it though and it doesn't seem to have
hit lkml, so I can't find the original patch...)