Re: [RFC PATCH 3/6] mm: page_owner: add support for splitting to any order in split page_owner.

[Matthew Wilcox]

On Tue, Nov 17, 2020 at 04:12:03PM -0500, Zi Yan wrote:
> On 17 Nov 2020, at 16:05, Matthew Wilcox wrote:
> 
> > On Fri, Nov 13, 2020 at 05:38:01PM -0800, Roman Gushchin wrote:
> >> On Fri, Nov 13, 2020 at 08:08:58PM -0500, Zi Yan wrote:
> >>> Matthew recently converted split_page_owner to take nr instead of order.[1]
> >>> But I am not
> >>> sure why, since it seems to me that two call sites (__split_huge_page in
> >>> mm/huge_memory.c and split_page in mm/page_alloc.c) can pass the order
> >>> information.
> >>
> >> Yeah, I'm not sure why too. Maybe Matthew has some input here?
> >> You can also pass new_nr, but IMO orders look so much better here.
> >
> > If only I'd written that information in the changelog ... oh wait, I did!
> >
> >     mm/page_owner: change split_page_owner to take a count
> >
> >     The implementation of split_page_owner() prefers a count rather than the
> >     old order of the page.  When we support a variable size THP, we won't
> >     have the order at this point, but we will have the number of pages.
> >     So change the interface to what the caller and callee would prefer.
> 
> There are two callers, split_page in mm/page_alloc.c and __split_huge_page in
> mm/huge_memory.c. The former has the page order. The latter has the page order
> information before __split_huge_page_tail is called, so we can do
> old_order = thp_order(head) instead of nr = thp_nr_page(head) and use old_order.
> What am I missing there?

Sure, we could also do that.  But what I wrote was true at the time I
wrote it.