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Message-ID: <ZuS0wKBUTSWvD_FZ@casper.infradead.org>
Date: Fri, 13 Sep 2024 22:55:12 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
Jesper Dangaard Brouer <hawk@...nel.org>,
Ilias Apalodimas <ilias.apalodimas@...aro.org>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Simon Horman <horms@...nel.org>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Linux Next Mailing List <linux-next@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH net-next v2] page_pool: fix build on powerpc with GCC 14
On Fri, Sep 13, 2024 at 09:33:51PM +0000, 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
>
> Root caused in this thread:
> https://lore.kernel.org/netdev/913e2fbd-d318-4c9b-aed2-4d333a1d5cf0@cs-soprasteria.com/
It would be better to include a direct link to the GCC bugzilla.
> 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@...nel.org>
> Cc: Stephen Rothwell <sfr@...b.auug.org.au>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: David Miller <davem@...emloft.net>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: Networking <netdev@...r.kernel.org>
> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
> Cc: Linux Next Mailing List <linux-next@...r.kernel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
> Cc: Matthew Wilcox <willy@...radead.org>
>
> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>
>
> ---
>
> v2: https://lore.kernel.org/netdev/20240913192036.3289003-1-almasrymina@google.com/
>
> - 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
>
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