On 12/9/20 6:33 AM, Matthew Wilcox wrote: > 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 >> Ugh. Yeah will fix. >>> 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? > Nop, my previous text was broken. > 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. > Thanks, I will use this instead. >>> 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.) >> Indeed one could use range_len() / pgmap->align and it would work. But (...) >> 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. :) >> My use of pfns to calculate the nr of pages was to remain consistent with the rest of the code in the function taking references in the pgmap->ref. The usages one sees ofrange_len are are when the hotplug takes place which work at addresses and not PFNs. >> 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. > If possible I would rather keep the pfns as with the rest of the code. Another alternative is like a range_nr_pages helper but I am not sure it's worth the trouble for one caller. Joao
