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Subject: Re: [PATCH] mm/pgtable/debug: Fix test validating architecture page
 table helpers
From:   Christophe Leroy <christophe.leroy@c-s.fr>
To:     Anshuman Khandual <anshuman.khandual@arm.com>, linux-mm@kvack.org
Cc:     Mark Rutland <mark.rutland@arm.com>, linux-ia64@vger.kernel.org,
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Date:   Fri, 13 Sep 2019 09:11:49 +0200
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Le 13/09/2019 à 09:03, Christophe Leroy a écrit :
> 
> 
> Le 13/09/2019 à 08:58, Anshuman Khandual a écrit :
>> On 09/13/2019 11:53 AM, Christophe Leroy wrote:
>>> Fix build failure on powerpc.
>>>
>>> Fix preemption imbalance.
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>> ---
>>>   mm/arch_pgtable_test.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/mm/arch_pgtable_test.c b/mm/arch_pgtable_test.c
>>> index 8b4a92756ad8..f2b3c9ec35fa 100644
>>> --- a/mm/arch_pgtable_test.c
>>> +++ b/mm/arch_pgtable_test.c
>>> @@ -24,6 +24,7 @@
>>>   #include <linux/swap.h>
>>>   #include <linux/swapops.h>
>>>   #include <linux/sched/mm.h>
>>> +#include <linux/highmem.h>
>>
>> This is okay.
>>
>>>   #include <asm/pgalloc.h>
>>>   #include <asm/pgtable.h>
>>> @@ -400,6 +401,8 @@ static int __init arch_pgtable_tests_init(void)
>>>       p4d_clear_tests(p4dp);
>>>       pgd_clear_tests(mm, pgdp);
>>> +    pte_unmap(ptep);
>>> +
>>
>> Now the preemption imbalance via pte_alloc_map() path i.e
>>
>> pte_alloc_map() -> pte_offset_map() -> kmap_atomic()
>>
>> Is not this very much powerpc 32 specific or this will be applicable
>> for all platform which uses kmap_XXX() to map high memory ?
>>
> 
> See 
> https://elixir.bootlin.com/linux/v5.3-rc8/source/include/linux/highmem.h#L91 
> 
> 
> I think it applies at least to all arches using the generic implementation.
> 
> Applies also to arm:
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/arm/mm/highmem.c#L52
> 
> Applies also to mips:
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/mips/mm/highmem.c#L47
> 
> Same on sparc:
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/sparc/mm/highmem.c#L52 
> 
> 
> Same on x86:
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/x86/mm/highmem_32.c#L34 
> 
> 
> I have not checked others, but I guess it is like that for all.
> 


Seems like I answered too quickly. All kmap_atomic() do 
preempt_disable(), but not all pte_alloc_map() call kmap_atomic().

However, for instance ARM does:

https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/arm/include/asm/pgtable.h#L200

And X86 as well:

https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/x86/include/asm/pgtable_32.h#L51

Microblaze also:

https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/microblaze/include/asm/pgtable.h#L495

Christophe