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Date:   Wed, 12 May 2021 17:26:12 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Eric Dumazet <edumazet@google.com>
Cc:     Ilias Apalodimas <ilias.apalodimas@linaro.org>,
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Subject: Re: [PATCH net-next v4 1/4] mm: add a signature in struct page
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On Wed, May 12, 2021 at 06:09:21PM +0200, Eric Dumazet wrote:
> On Wed, May 12, 2021 at 6:03 PM Matthew Wilcox <willy@infradead.org> 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@infradead.org>
> > 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@infradead.org>
> > ---
> >  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 */
> 
> This would break compound_head() ?

No, compound_head() only checks bit 0.  If bit 0 is clear, then this is
not a tail page.