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

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Message-ID: <791773e4-9590-4093-a916-a02981633d16@arm.com>
Date: Wed, 7 Jan 2026 09:13:54 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Anshuman Khandual <anshuman.khandual@....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 07/01/2026 09:07, Anshuman Khandual wrote:
> 
> 
> 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 ?

I don't see the value of it myself, given the PMD assert is already doing the
required check.

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