On Thu, Mar 2, 2017 at 3:52 PM, Shaohua Li <shli@xxxxxxxxxx> wrote: > On Thu, Mar 02, 2017 at 10:34:25AM +0800, Ming Lei wrote: >> Hi Shaohua, >> >> On Wed, Mar 1, 2017 at 7:42 AM, Shaohua Li <shli@xxxxxxxxxx> wrote: >> > On Tue, Feb 28, 2017 at 11:41:39PM +0800, Ming Lei wrote: >> >> Use this helper, instead of direct access to .bi_vcnt. >> > >> > what We really need to do for the behind IO is: >> > - allocate memory and copy bio data to the memory >> > - let behind bio do IO against the memory >> > >> > The behind bio doesn't need to have the exactly same bio_vec setting. If we >> > just track the new memory, we don't need use the bio_segments_all and access >> > bio_vec too. >> >> But we need to figure out how many vecs(each vec store one page) to be >> allocated for the cloned/behind bio, and that is the only value of >> bio_segments_all() here. Or you have idea to avoid that? > > As I said, the behind bio doesn't need to have the exactly same bio_vec > setting. We just allocate memory and copy original bio data to the memory, > then do IO against the new memory. The behind bio > segments == (bio->bi_iter.bi_size + PAGE_SIZE - 1) >> PAGE_SHIFT The equation isn't always correct, especially when bvec includes just part of page, and it is quite often in case of mkfs, in which one bvec often includes 512byte buffer. Thanks, Ming Lei
