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Date:   Sat, 5 Jun 2021 15:32:20 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Matteo Croce <mcroce@linux.microsoft.com>
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Subject: Re: [PATCH net-next v7 1/5] mm: add a signature in struct page
Message-ID: <YLuK9P+loeKwUUK3@casper.infradead.org>
References: <20210604183349.30040-1-mcroce@linux.microsoft.com>
 <20210604183349.30040-2-mcroce@linux.microsoft.com>
 <YLp6D7mEh85vL+pY@casper.infradead.org>
 <CAFnufp2jGRsr9jexBLFRZfJu9AwGO0ghzExT1R4bJdscwHqSnQ@mail.gmail.com>
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On Sat, Jun 05, 2021 at 12:59:50AM +0200, Matteo Croce wrote:
> On Fri, Jun 4, 2021 at 9:08 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Fri, Jun 04, 2021 at 08:33:45PM +0200, Matteo Croce wrote:
> > > @@ -130,7 +137,10 @@ struct page {
> > >                       };
> > >               };
> > >               struct {        /* Tail pages of compound page */
> > > -                     unsigned long compound_head;    /* Bit zero is set */
> > > +                     /* Bit zero is set
> > > +                      * Bit one if pfmemalloc page
> > > +                      */
> > > +                     unsigned long compound_head;
> >
> > I would drop this hunk.  Bit 1 is not used for this purpose in tail
> > pages; it's used for that purpose in head and base pages.
> >
> > I suppose we could do something like ...
> >
> >  static inline void set_page_pfmemalloc(struct page *page)
> >  {
> > -       page->index = -1UL;
> > +       page->lru.next = (void *)2;
> >  }
> >
> > if it's causing confusion.
> >
> 
> If you prefer, ok for me.
> Why not "(void *)BIT(1)"? Just to remark that it's a single bit and
> not a magic like value?

I don't have a strong preference.  I'd use '2', but I wouldn't ask
BIT(1) to be changed.