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

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Message-ID: <30e8dee7-e98e-42cb-aab3-6b75f1a6316d@csgroup.eu>
Date: Sat, 14 Sep 2024 08:50:46 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Matthew Wilcox <willy@...radead.org>,
 Mina Almasry <almasrymina@...gle.com>
Cc: 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

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.

Christophe