Re: [PATCH] block: Make __bio_clone_fast() copy bi_vcnt
From: Jens Axboe <axboe@kernel.dk>
Date: 2018-06-29 02:18:27
On 6/28/18 5:16 PM, Kent Overstreet wrote:
On Fri, Jun 29, 2018 at 07:10:47AM +0800, Ming Lei wrote:quoted
On Thu, Jun 28, 2018 at 11:21 PM, Bart Van Assche [off-list ref] wrote:quoted
On 06/27/18 17:30, Ming Lei wrote:quoted
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 particularquoted
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...)
Yeah, you are both right, I was smoking crack. -- Jens Axboe