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.
