linux-kernel - Re: [PATCH 1/3] mm,hugetlb: use folio fields in second tail page

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Date:   Thu, 3 Nov 2022 21:29:02 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Sidhartha Kumar <sidhartha.kumar@...cle.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Mina Almasry <almasrymina@...gle.com>,
        James Houghton <jthoughton@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] mm,hugetlb: use folio fields in second tail page

On Thu, 3 Nov 2022, Sidhartha Kumar wrote:
> On 11/2/22 6:48 PM, Hugh Dickins wrote:
...
> > Undo "mm: add private field of first tail to struct page and struct
> > folio"'s recent addition of private_1 to the folio tail: instead add
> > hugetlb_subpool, hugetlb_cgroup, hugetlb_cgroup_rsvd, hugetlb_hwpoison
> > to a second tail page of the folio: THP has long been using several
> > fields of that tail, so make better use of it for hugetlb too.
> > This is not how a generic folio should be declared in future,
> > but it is an effective transitional way to make use of it.
...
> > @@ -260,13 +267,16 @@ struct page {
> >    *    to find how many references there are to this folio.
> >    * @memcg_data: Memory Control Group data.
> >    * @_flags_1: For large folios, additional page flags.
> > - * @__head: Points to the folio.  Do not use.
> > + * @_head_1: Points to the folio.  Do not use.
> 
> Changes to my original patch set look good, this seems to be a cleaner
> implementation.

Thanks a lot, Sidhartha, I'm glad to hear that it works for you too.

I expect that it will be done differently in the future: maybe generalizing
the additional fields to further "private"s as you did, letting different
subsystems accessorize them differently; or removing them completely from
struct folio, letting subsystems declare their own struct folio containers.
I don't know how that will end up, but this for now seems good and clear.

> 
> Should the usage of page_1 and page_2 also be documented here?

You must have something interesting in mind to document about them,
but I cannot guess what! They are for field alignment, not for use.
(page_2 to help when/if someone needs to add another pageful.)

Do you mean that I should copy the 
	/* private: the union with struct page is transitional */
comment from above the original "struct page page;" line I copied?
Or give all three of them a few underscores to imply not for use?

Thanks,
Hugh