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

From: Mina Almasry <[email-masked]>
Date: Fri, 13 Sep 2024 19:20:36 +0000

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/[email-masked]/

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 <[email-masked]>
Cc: Stephen Rothwell <[email-masked]>
Cc: Jakub Kicinski <[email-masked]>
Cc: David Miller <[email-masked]>
Cc: Paolo Abeni <[email-masked]>
Cc: Networking <[email-masked]>
Cc: Linux Kernel Mailing List <[email-masked]>
Cc: Linux Next Mailing List <[email-masked]>
Cc: Arnd Bergmann <[email-masked]>
Cc: "[email-masked]" <[email-masked]>

Suggested-by: LEROY Christophe <[email-masked]>
Signed-off-by: Mina Almasry <[email-masked]>

---
 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