[PATCH net-next v1] mm: fix build on powerpc with GCC 14

[Mina Almasry]

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

Causing an unaligned read error. Fix this by bitwise operand instead of
an arthimetic operation to clear the pointer, which probably
communicates the intention of the code a bit better anyway.

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>

Suggested-by: LEROY Christophe <christophe.leroy2@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>

---
 include/linux/page-flags.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5769fe6e4950..ea4005d2d1a9 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -239,8 +239,8 @@ 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;
+	if (unlikely(head & 1UL))
+		return head & ~1UL;
 	return (unsigned long)page_fixed_fake_head(page);
 }
 
-- 
2.46.0.662.g92d0881bb0-goog