linux-kernel - Re: [RFC PATCH v2 10/20] powerpc/mm: Fix __find_linux

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Date: Sat, 25 May 2024 06:41:46 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Oscar Salvador <osalvador@...e.de>
CC: Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe
	<jgg@...dia.com>, Peter Xu <peterx@...hat.com>, Michael Ellerman
	<mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH v2 10/20] powerpc/mm: Fix __find_linux_pte() on 32
 bits with PMD leaf entries



Le 25/05/2024 à 06:12, Oscar Salvador a écrit :
> On Fri, May 17, 2024 at 09:00:04PM +0200, Christophe Leroy wrote:
>> Building on 32 bits with pmd_leaf() not returning always false leads
>> to the following error:
> 
> I am curious though.
> pmd_leaf is only defined in include/linux/pgtable.h for 32bits, and is hardcoded
> to false.
> I do not see where we change it in previous patches, so is this artificial?

Patch 17 brings pmd_leaf()

> 
>>
>>    CC      arch/powerpc/mm/pgtable.o
>> arch/powerpc/mm/pgtable.c: In function '__find_linux_pte':
>> arch/powerpc/mm/pgtable.c:506:1: error: function may return address of local variable [-Werror=return-local-addr]
>>    506 | }
>>        | ^
>> arch/powerpc/mm/pgtable.c:394:15: note: declared here
>>    394 |         pud_t pud, *pudp;
>>        |               ^~~
>> arch/powerpc/mm/pgtable.c:394:15: note: declared here
>>
>> This is due to pmd_offset() being a no-op in that case.
> 
> This is because 32bits powerpc include pgtable-nopmd.h?
> 
>> So rework it for powerpc/32 so that pXd_offset() are used on real
>> pointers and not on on-stack copies.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>   arch/powerpc/mm/pgtable.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
>> index 59f0d7706d2f..51ee508eeb5b 100644
>> --- a/arch/powerpc/mm/pgtable.c
>> +++ b/arch/powerpc/mm/pgtable.c
>> @@ -390,8 +390,12 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
>>   			bool *is_thp, unsigned *hpage_shift)
>>   {
>>   	pgd_t *pgdp;
>> -	p4d_t p4d, *p4dp;
>> -	pud_t pud, *pudp;
>> +	p4d_t *p4dp;
>> +	pud_t *pudp;
>> +#ifdef CONFIG_PPC64
>> +	p4d_t p4d;
>> +	pud_t pud;
>> +#endif
>>   	pmd_t pmd, *pmdp;
>>   	pte_t *ret_pte;
>>   	hugepd_t *hpdp = NULL;
>> @@ -412,6 +416,7 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
>>   	 */
>>   	pgdp = pgdir + pgd_index(ea);
>>   	p4dp = p4d_offset(pgdp, ea);
>> +#ifdef CONFIG_PPC64
>>   	p4d  = READ_ONCE(*p4dp);
>>   	pdshift = P4D_SHIFT;
>>   
>> @@ -452,6 +457,11 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
>>   
>>   	pdshift = PMD_SHIFT;
>>   	pmdp = pmd_offset(&pud, ea);
>> +#else
>> +	p4dp = p4d_offset(pgdp, ea);
>> +	pudp = pud_offset(p4dp, ea);
>> +	pmdp = pmd_offset(pudp, ea);
> 
> I would drop a comment on top explaining that these are no-op for 32bits,
> otherwise it might not be obvious to people as why this distiction between 64 and
> 32bits.

Ok

> 
> Other than that looks good to me
> 
>   
>