On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote: > On 20/11/2020 01:20, Matthew Wilcox wrote: > > On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote: > >> The block layer spends quite a while in iov_iter_npages(), but for the > >> bvec case the number of pages is already known and stored in > >> iter->nr_segs, so it can be returned immediately as an optimisation > > > > Er ... no, it doesn't. nr_segs is the number of bvecs. Each bvec can > > store up to 4GB of contiguous physical memory. > > Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a > stupid statement. Thanks! > > Are there many users of that? All these iterators are a huge burden, > just to count one 4KB page in bvec it takes 2% of CPU time for me. __bio_try_merge_page() will create multipage BIOs, and that's called from a number of places including bio_try_merge_hw_seg(), bio_add_page(), and __bio_iov_iter_get_pages() so ... yeah, it's used a lot.
