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

[Matthew Wilcox <willy@infradead.org>]

On Tue, Nov 17, 2020 at 01:35:37PM -0800, Roman Gushchin wrote:
> On Tue, Nov 17, 2020 at 09:22:55PM +0000, Matthew Wilcox wrote:
> > 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.
> 
> Sure, I was asking about if you're ok with going back to orders or there are better
> ideas. I'm sorry if it wasn't clear and sounded differently.
> 
> It just seems to me than a function is taking nr and order (as in Zi's last version),
> I'd expect that it's a number of pages of given order, or something like this.
> So I'd avoid mixing them. Orders are slightly better if nr is always a power of two,
> it's just more obvious from looking at the code.

I think it's awkward no matter which way round we do it.

If we pass old_order, new_order then we create extra work for both caller
and callee.

If we pass old_nr, new_order, it looks weird for humans.

At the end of the day, I'm not that invested in which we do.