linux-kernel - Re: [PATCH] arm64/mm: Assert NR_BM_PUD_TABLES for bm

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Message-ID: <32030401-06d3-40ad-a0b6-fbd366092f2b@arm.com>
Date: Wed, 7 Jan 2026 14:37:51 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>, linux-arm-kernel@...ts.infradead.org
Cc: mark.rutland@....com, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Assert NR_BM_PUD_TABLES for bm_pud[]



On 07/01/26 2:03 PM, Ryan Roberts wrote:
> On 07/01/2026 06:16, Anshuman Khandual wrote:
>> There is an implicit assumption about bm_pud[] being single entry PUD table
>> just like bp_pmd[] which is a single entry PMD table. Though only the later
>> gets asserted via NR_BM_PMD_TABLES. Hence let's add a similar check for PUD
>> table as well.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  arch/arm64/mm/fixmap.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
>> index c5c5425791da..e02594be8b8e 100644
>> --- a/arch/arm64/mm/fixmap.c
>> +++ b/arch/arm64/mm/fixmap.c
>> @@ -23,8 +23,11 @@ static_assert(FIXADDR_TOT_START > PCI_IO_END);
>>  	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PMD_SHIFT)
>>  #define NR_BM_PMD_TABLES \
>>  	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PUD_SHIFT)
>> +#define NR_BM_PUD_TABLES \
>> +	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, P4D_SHIFT)
>>  
>>  static_assert(NR_BM_PMD_TABLES == 1);
>> +static_assert(NR_BM_PUD_TABLES == 1);
> 
> But if there is only one PMD table, there must only be one PUD table (and one
> P4D table, and one PGD table) by definition. So I don't think this is checking
> anything that the existing PMD table check is not.

Agreed but should not this check help tighten this code up given that bm_pud[]
array gets used for possible PUD entries ?

> 
> Thanks,
> Ryan
> 
>>  
>>  #define __BM_TABLE_IDX(addr, shift) \
>>  	(((addr) >> (shift)) - (FIXADDR_TOT_START >> (shift)))
>