linux-kernel - Re: [PATCH] mm: Remove PMD alignment constraint in execmem

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Message-ID: <c468ffc7-bf4f-4b0e-b02f-f926f1c2a836@arm.com>
Date: Mon, 22 Sep 2025 12:20:24 +0530
From: Dev Jain <dev.jain@....com>
To: Anshuman Khandual <anshuman.khandual@....com>, akpm@...ux-foundation.org
Cc: rppt@...nel.org, ryan.roberts@....com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Remove PMD alignment constraint in execmem_vmalloc()


On 22/09/25 11:32 am, Anshuman Khandual wrote:
>
> On 18/09/25 3:04 PM, Dev Jain wrote:
>> When using vmalloc with VM_ALLOW_HUGE_VMAP flag, it will set the alignment
>> to PMD_SIZE internally, if it deems huge mappings to be eligible.
>> Therefore, setting the alignment in execmem_vmalloc is redundant. Apart
>> from this, it also reduces the probability of allocation in case vmalloc
>> fails to allocate hugepages - in the fallback case, vmalloc tries to use
>> the original alignment and allocate basepages, which unfortunately will
>> again be PMD_SIZE passed over from execmem_vmalloc, thus constraining
>> the search for a free space in vmalloc region.
>>
>> Therefore, remove this constraint.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>> mm-selftests pass, but I am not sure if they touch execmem code, and I
>> have no experience with this code.
>>
>>   mm/execmem.c | 3 ---
>>   1 file changed, 3 deletions(-)
>>
>> diff --git a/mm/execmem.c b/mm/execmem.c
>> index 0822305413ec..810a4ba9c924 100644
>> --- a/mm/execmem.c
>> +++ b/mm/execmem.c
>> @@ -38,9 +38,6 @@ static void *execmem_vmalloc(struct execmem_range *range, size_t size,
>>   	if (kasan)
>>   		vm_flags |= VM_DEFER_KMEMLEAK;
>>   
>> -	if (vm_flags & VM_ALLOW_HUGE_VMAP)
>> -		align = PMD_SIZE;
>> -
> So if the above assignment is getting dropped here, probably the local
> variable 'align' could be dropped as well and range->alignment be used
> directly instead ?

Sure, but that isn't a big deal. Replacing with range->aligment will
push the arguments declaration into an extra line in __vmalloc_node_range.
So will prefer not respinning for this triviality, this has already
been pulled into mm-new :)

>>   	p = __vmalloc_node_range(size, align, start, end, gfp_flags,
>>   				 pgprot, vm_flags, NUMA_NO_NODE,
>>   				 __builtin_return_address(0));