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

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Message-ID: <Zua6qtUCG84236gp@casper.infradead.org>
Date: Sun, 15 Sep 2024 11:44:58 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Simon Horman <horms@...nel.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Jakub Kicinski <kuba@...nel.org>,
	David Miller <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
	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 v1] mm: fix build on powerpc with GCC 14

On Sat, Sep 14, 2024 at 08:50:46AM +0200, Christophe Leroy wrote:
> Hi,
> 
> Le 13/09/2024 à 21:22, Matthew Wilcox a écrit :
> > On Fri, Sep 13, 2024 at 07:20:36PM +0000, Mina Almasry wrote:
> > > +++ 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);
> > 
> > NAK, that pessimises compound_head().
> > 
> 
> Can you please give more details on what the difference is ?
> 
> I can't see what it pessimises. In both cases, you test if the value is odd,
> when it is odd you make it even.

On x86, for example, it is perfectly valid to load a 64-bit value from
an offset of 0x2f relative to a pointer.  So there's no need to make it
even.