Re: [PATCH net-next v2] page_pool: fix build on powerpc with GCC 14

[Mina Almasry]

On Fri, Sep 13, 2024 at 3:27 PM Stanislav Fomichev <stfomichev@xxxxxxxxx> wrote:
>
> On 09/13, Matthew Wilcox wrote:
> > On Fri, Sep 13, 2024 at 02:55:19PM -0700, Stanislav Fomichev wrote:
> > > On 09/13, Mina Almasry wrote:
> > > > Building net-next with powerpc with GCC 14 compiler results in this
> > > > build error:
> > > >
> > > > /home/sfr/next/tmp/ccuSzwiR.s: Assembler messages:
> > > > /home/sfr/next/tmp/ccuSzwiR.s:2579: Error: operand out of domain (39 is
> > > > not a multiple of 4)
> > > > make[5]: *** [/home/sfr/next/next/scripts/Makefile.build:229:
> > > > net/core/page_pool.o] Error 1
> > >
> > > Are we sure this is the only place where we can hit by this?
> >
> > It's a compilation error, so yes, we're sure.
>
> We also have netmem_compound_head() which does page_to_netmem(compound_head()).
> Wondering whether we'll eventually hit a similar issue over there.

A bit of a head scratcher why the compiler isn't running into the same
issue for netmem_compound_head.

The callsites of netmem_compound_head are in net/core/skbuff.c, in
skb_pp_frag_ref & napi_pp_put_page. Looking at the assembly generated,
looks like somehow the compiler completely optimized out the call in
napi_pp_put_page, and the call in skb_pp_frag_ref morphs into:

 # net/core/skbuff.c:1047:      return
napi_pp_put_page(page_to_netmem(virt_to_page(data)));
        addis 9,2,.LC63@toc@ha   # tmp158,,
        ld 10,.LC63@toc@l(9)     #, tmp140
 # ./arch/powerpc/include/asm/page.h:230:       return __pa(kaddr) >>
PAGE_SHIFT;
        rldicl 9,31,48,20        #, _17, head,
 # net/core/skbuff.c:1047:      return
napi_pp_put_page(page_to_netmem(virt_to_page(data)));
        sldi 9,9,6       #, _18, _17
 # net/core/skbuff.c:1047:      return
napi_pp_put_page(page_to_netmem(virt_to_page(data)));
        ld 3,0(10)       # vmemmap, vmemmap
 # net/core/skbuff.c:1047:      return
napi_pp_put_page(page_to_netmem(virt_to_page(data)));
        add 3,3,9        #, vmemmap, _18

Since it's page_to_netmem(virt_to_page(data)) (not virt_to_head_page),
the we don't hit there right now. It's certainly possible to trigger
this in the future.

I think we could also READ_ONCE in netmem_compound_head for some
future proofness.

--
Thanks,
Mina