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.vanass...@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...)
