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From:   Arjun Roy <arjunroy@google.com>
Date:   Thu, 27 Feb 2020 09:44:54 -0800
Message-ID: <CAOFY-A2CFi0pX1BBsuruntk0AM9doseCMnFCJi192BYojaBUUw@mail.gmail.com>
Subject: Re: linux-next: build failure after merge of the akpm tree
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Stephen Rothwell <sfr@canb.auug.org.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Soheil Hassas Yeganeh <soheil@google.com>
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On Thu, Feb 27, 2020 at 9:13 AM Arjun Roy <arjunroy@google.com> wrote:
>
> On Thu, Feb 27, 2020 at 1:03 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >
> > Hi Stephen et al,
> >
> > On Thu, Feb 27, 2020 at 5:12 AM Stephen Rothwell <sfr@canb.auug.org.au> wrote:
> > > After merging the akpm tree, today's linux-next build (sparc defconfig)
> > > failed like this:
> > >
> > > In file included from include/linux/list.h:9:0,
> > >                  from include/linux/smp.h:12,
> > >                  from include/linux/kernel_stat.h:5,
> > >                  from mm/memory.c:42:
> > > mm/memory.c: In function 'insert_pages':
> > > mm/memory.c:1523:41: error: implicit declaration of function 'pte_index'; did you mean 'page_index'? [-Werror=implicit-function-declaration]
> > >    remaining_pages_total, PTRS_PER_PTE - pte_index(addr));
> > >                                          ^
> > > include/linux/kernel.h:842:40: note: in definition of macro '__typecheck'
> > >    (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
> > >                                         ^
> > > include/linux/kernel.h:866:24: note: in expansion of macro '__safe_cmp'
> > >   __builtin_choose_expr(__safe_cmp(x, y), \
> > >                         ^~~~~~~~~~
> > > include/linux/kernel.h:934:27: note: in expansion of macro '__careful_cmp'
> > >  #define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
> > >                            ^~~~~~~~~~~~~
> > > mm/memory.c:1522:26: note: in expansion of macro 'min_t'
> > >   pages_to_write_in_pmd = min_t(unsigned long,
> > >                           ^~~~~
> >
> > Same issue on m68k, as per a report from kisskb.
> >
> > > Caused by patch
> > >
> > >   "mm/memory.c: add vm_insert_pages()"
> > >
> > > sparc32 does not implement pte_index at all :-(
> >
> > Seems like about only half of the architectures do.
> >
>
> :/ I begin to suspect the only sane way to make this work is to have a
> per-arch header defined method, returning a bool saying whether
> pte_index() is meaningful or not on that arch, and early on in
> vm_insert_pages() if that bool returns true, to just call
> vm_insert_page() in a loop.
>

So, here is what I propose: something like the following macro in a
per-arch header:

#define PTE_INDEX_DEFINED 1 // or 0 if it is not

In mm/memory.c, another macro:

#ifndef PTE_INDEX_DEFINED
#define PTE_INDEX_DEFINED 0
#endifndef

And inside vm_insert_pages:

int vm_insert_pages() {

#if PTE_INDEX_DEFINED

// The existing method

#else

for (i=0; i<n; ++i)
        vm_insert_page(i)

#endif
}

That way:
1. No playing whack-a-mole with different architectures
2. Architecture that knows pte_index is meaningful works can define
this explicitly
3. Can remove the sparc patches modifying pte_index that Stephen and I
contributed.

If that sounds acceptable I can cook a patch.

Thanks,
-Arjun

> -Arjun
>
> > Gr{oetje,eeting}s,
> >
> >                         Geert
> >
> > --
> > Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
> >
> > In personal conversations with technical people, I call myself a hacker. But
> > when I'm talking to journalists I just say "programmer" or something like that.
> >                                 -- Linus Torvalds