On Tue, Dec 08, 2020 at 09:59:19PM -0800, John Hubbard wrote: > On 12/8/20 9:28 AM, Joao Martins wrote: > > Add a new flag for struct dev_pagemap which designates that a a pagemap > > a a > > > is described as a set of compound pages or in other words, that how > > pages are grouped together in the page tables are reflected in how we > > describe struct pages. This means that rather than initializing > > individual struct pages, we also initialize these struct pages, as > > Let's not say "rather than x, we also do y", because it's self-contradictory. > I think you want to just leave out the "also", like this: > > "This means that rather than initializing> individual struct pages, we > initialize these struct pages ..." > > Is that right? I'd phrase it as: Add a new flag for struct dev_pagemap which specifies that a pagemap is composed of a set of compound pages instead of individual pages. When these pages are initialised, most are initialised as tail pages instead of order-0 pages. > > For certain ZONE_DEVICE users, like device-dax, which have a fixed page > > size, this creates an opportunity to optimize GUP and GUP-fast walkers, > > thus playing the same tricks as hugetlb pages. Rather than "playing the same tricks", how about "are treated the same way as THP or hugetlb pages"? > > + if (pgmap->flags & PGMAP_COMPOUND) > > + percpu_ref_get_many(pgmap->ref, (pfn_end(pgmap, range_id) > > + - pfn_first(pgmap, range_id)) / PHYS_PFN(pgmap->align)); > > Is there some reason that we cannot use range_len(), instead of pfn_end() minus > pfn_first()? (Yes, this more about the pre-existing code than about your change.) > > And if not, then why are the nearby range_len() uses OK? I realize that range_len() > is simpler and skips a case, but it's not clear that it's required here. But I'm > new to this area so be warned. :) > > Also, dividing by PHYS_PFN() feels quite misleading: that function does what you > happen to want, but is not named accordingly. Can you use or create something > more accurately named? Like "number of pages in this large page"? We have compound_nr(), but that takes a struct page as an argument. We also have HPAGE_NR_PAGES. I'm not quite clear what you want.
