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

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Message-ID: <4f2005ba-ff12-4ad9-bd4c-33cae87bdc78@arm.com>
Date: Thu, 8 Jan 2026 09:34:00 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Ryan Roberts <ryan.roberts@....com>,
 linux-arm-kernel@...ts.infradead.org,
 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 3:41 PM, Mark Rutland wrote:
> On Wed, Jan 07, 2026 at 02:37:51PM +0530, 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 ?
> 
> An assert isn't going to help with code generation, so this patch alone is not
> useful. When I last looked at actually ommiting the table in various
> configurations, it was more trouble than it was worth.
> 
> As Ryan said, we deliberately only added the assert for the PMD level,
> and it's sufficient as-is. That happened in commit:
> 
>   414c109bdf49 ("arm64: mm: always map fixmap at page granularity")
> 
> ... where the commit message said:
> 
>     Our VA layout means that the fixmap will always fall within a single PMD
>     table (and consequently, within a single PUD/P4D/PGD entry), which we
>     can verify at compile time with a static_assert(). With that assert a
>     number of runtime warnings become impossible, and are removed.
> 
> Please leave this as-is.

Alright, makes sense.