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

[Mina Almasry]

On Fri, Sep 13, 2024 at 2:33 PM Mina Almasry <almasrymina@xxxxxxxxxx> 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
>
> Root caused in this thread:
> https://lore.kernel.org/netdev/913e2fbd-d318-4c9b-aed2-4d333a1d5cf0@xxxxxxxxxxxxxxxxxx/
>
> We try to access offset 40 in the pointer returned by this function:
>
> static inline unsigned long _compound_head(const struct page *page)
> {
>         unsigned long head = READ_ONCE(page->compound_head);
>
>         if (unlikely(head & 1))
>                 return head - 1;
>         return (unsigned long)page_fixed_fake_head(page);
> }
>
> The GCC 14 (but not 11) compiler optimizes this by doing:
>
> ld page + 39
>
> Rather than:
>
> ld (page - 1) + 40
>
> And causing an unaligned load. Get around this by issuing a READ_ONCE as
> we convert the page to netmem.  That disables the compiler optimizing the
> load in this way.
>
> Cc: Simon Horman <horms@xxxxxxxxxx>
> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: David Miller <davem@xxxxxxxxxxxxx>
> Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> Cc: Networking <netdev@xxxxxxxxxxxxxxx>
> Cc: Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
> Cc: Linux Next Mailing List <linux-next@xxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: "linuxppc-dev@xxxxxxxxxxxxxxxx" <linuxppc-dev@xxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>
> Suggested-by: Jakub Kicinski <kuba@xxxxxxxxxx>
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>

Gah, right after I hit send I realized I missed the 24hr rule.
Although I'm unsure about the urgency of build fixes. Sorry about
that.

-- 
Thanks,
Mina