Re: [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages

[Matthew Wilcox <willy@infradead.org>]

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.