Re: [PATCH net-next v4 1/4] mm: add a signature in struct page

[Eric Dumazet]

On Wed, May 12, 2021 at 6:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, May 11, 2021 at 05:25:36PM +0300, Ilias Apalodimas wrote:
> > Nope not at all, either would work. we'll switch to that
>
> You'll need something like this because of the current use of
> page->index to mean "pfmemalloc".
>
> From ecd6d912056a21bbe55d997c01f96b0b8b9fbc31 Mon Sep 17 00:00:00 2001
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Date: Fri, 16 Apr 2021 18:12:33 -0400
> Subject: [PATCH] mm: Indicate pfmemalloc pages in compound_head
>
> The net page_pool wants to use a magic value to identify page pool pages.
> The best place to put it is in the first word where it can be clearly a
> non-pointer value.  That means shifting dma_addr up to alias with ->index,
> which means we need to find another way to indicate page_is_pfmemalloc().
> Since page_pool doesn't want to set its magic value on pages which are
> pfmemalloc, we can use bit 1 of compound_head to indicate that the page
> came from the memory reserves.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  include/linux/mm.h       | 12 +++++++-----
>  include/linux/mm_types.h |  7 +++----
>  2 files changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bd21864449bf..4f9b2007efad 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1670,10 +1670,12 @@ struct address_space *page_mapping(struct page *page);
>  static inline bool page_is_pfmemalloc(const struct page *page)
>  {
>         /*
> -        * Page index cannot be this large so this must be
> -        * a pfmemalloc page.
> +        * This is not a tail page; compound_head of a head page is unused
> +        * at return from the page allocator, and will be overwritten
> +        * by callers who do not care whether the page came from the
> +        * reserves.
>          */
> -       return page->index == -1UL;
> +       return page->compound_head & 2;
>  }
>
>  /*
> @@ -1682,12 +1684,12 @@ static inline bool page_is_pfmemalloc(const struct page *page)
>   */
>  static inline void set_page_pfmemalloc(struct page *page)
>  {
> -       page->index = -1UL;
> +       page->compound_head = 2;
>  }
>
>  static inline void clear_page_pfmemalloc(struct page *page)
>  {
> -       page->index = 0;
> +       page->compound_head = 0;
>  }
>
>  /*
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5aacc1c10a45..1352e278939b 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -96,10 +96,9 @@ struct page {
>                         unsigned long private;
>                 };
>                 struct {        /* page_pool used by netstack */
> -                       /**
> -                        * @dma_addr: might require a 64-bit value on
> -                        * 32-bit architectures.
> -                        */
> +                       unsigned long pp_magic;
> +                       struct page_pool *pp;
> +                       unsigned long _pp_mapping_pad;
>                         unsigned long dma_addr[2];
>                 };
>                 struct {        /* slab, slob and slub */
> --
> 2.30.2
>

This would break compound_head() ?