On Fri, Mar 3, 2017 at 10:20 AM, Ming Lei <tom.leim...@gmail.com> wrote: > On Thu, Mar 2, 2017 at 3:52 PM, Shaohua Li <s...@kernel.org> 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 <s...@kernel.org> 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.
Think it further, your idea could be workable and more clean, but the change can be a bit big, looks we need to switch handling write behind into the following way: 1) replace bio_clone_bioset_partial() with bio_allocate(nr_vecs), and 'nr_vecs' is computed with your equation; 2) allocate 'nr_vecs' pages once and share them among all created bio in 1) 3) for each created bio, add each page into the bio via bio_add_page() 4) only for the 1st created bio, call bio_copy_data() to copy data from master bio. Let me know if you are OK with the above implementaion. Thanks, Ming Lei
