On Thu, Jun 28, 2018 at 04:54:44PM -0700, Bart Van Assche wrote: > On 06/28/18 16:16, Kent Overstreet wrote: > > 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@xxxxxxx> 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...) > > Hello Kent, > > Thanks for chiming in. The linux-block mailing list is archived by multiple > websites. The entire e-mail thread is available on e.g. > https://www.mail-archive.com/linux-block@xxxxxxxxxxxxxxx/msg23006.html. > > I have a question for you: at least in kernel v4.17 bio_clone_bioset() > copies bi_vcnt from the source to the destination bio. However, > __bio_clone_fast() doesn't copy bi_vcnt. Isn't that an inconsistency? No - when you use bio_clone_bioset() you get a bio that you own and can do whatever you want with, so it does make sense for it to initialize bi_vcnt. e.g. you could use bio_clone_bioset() when you're going to be bouncing a bio, iterating over each bvec and allocating a new page and copying data from the old page to the new page.
