[PATCH net-next v2] page_pool: 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

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>

---

v2: https://lore.kernel.org/netdev/20240913192036.3289003-1-almasrymina@xxxxxxxxxx/

- Work around this issue as we convert the page to netmem, instead of
  a generic change that affects compound_head().
---
 net/core/page_pool.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index a813d30d2135..74ea491d0ab2 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -859,12 +859,25 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data,
 {
 	int i, bulk_len = 0;
 	bool allow_direct;
+	netmem_ref netmem;
+	struct page *page;
 	bool in_softirq;
 
 	allow_direct = page_pool_napi_local(pool);
 
 	for (i = 0; i < count; i++) {
-		netmem_ref netmem = page_to_netmem(virt_to_head_page(data[i]));
+		page = virt_to_head_page(data[i]);
+
+		/* GCC 14 powerpc compiler will optimize reads into the
+		 * resulting netmem_ref into unaligned reads as it sees address
+		 * arithmetic in _compound_head() call that the page has come
+		 * from.
+		 *
+		 * The READ_ONCE here gets around that by breaking the
+		 * optimization chain between the address arithmetic and later
+		 * indexing.
+		 */
+		netmem = page_to_netmem(READ_ONCE(page));
 
 		/* It is not the last user for the page frag case */
 		if (!page_pool_is_last_ref(netmem))
-- 
2.46.0.662.g92d0881bb0-goog