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

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