linux-kernel - Re: [PATCH 3/3] cma: move allocation from HIGHMEM to a helper function

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Message-ID: <9b4e9e08-9849-4d55-a0b3-639439c35559@redhat.com>
Date: Thu, 3 Jul 2025 13:14:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Alexandre Ghiti <alexghiti@...osinc.com>, Pratyush Yadav
 <ptyadav@...zon.de>, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/3] cma: move allocation from HIGHMEM to a helper
 function

On 03.07.25 11:53, Oscar Salvador wrote:
> On Wed, Jul 02, 2025 at 08:36:05PM +0300, Mike Rapoport wrote:
>> From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
>>
>> When CONFIG_HIGMEM is enabled, __cma_declare_contiguous_nid() first
>> tries to allocate the area from HIGHMEM and if that fails it falls back
>> to allocation from low memory.
>>
>> Split allocation from HIGMEM into a helper function to further decouple
>> logic related to CONFIG_HIGHMEM.
>>
>> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
>> ---
>>   mm/cma.c | 52 +++++++++++++++++++++++++++++-----------------------
>>   1 file changed, 29 insertions(+), 23 deletions(-)
>>
>> diff --git a/mm/cma.c b/mm/cma.c
>> index 1df8ff312d99..0a24c46f3296 100644
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -376,6 +376,30 @@ static int __init cma_fixed_reserve(phys_addr_t base, phys_addr_t size)
>>   	return 0;
>>   }
>>   
>> +static phys_addr_t __init cma_alloc_highmem(phys_addr_t base, phys_addr_t size,
>> +			phys_addr_t align, phys_addr_t *limit, int nid)
>> +{
>> +	phys_addr_t addr = 0;
>> +
>> +	if (IS_ENABLED(CONFIG_HIGHMEM)) {
>> +		phys_addr_t highmem = __pa(high_memory - 1) + 1;
>> +
>> +		/*
>> +		 * All pages in the reserved area must come from the same zone.
>> +		 * If the requested region crosses the low/high memory boundary,
>> +		 * try allocating from high memory first and fall back to low
>> +		 * memory in case of failure.
>> +		 */
>> +		if (base < highmem && *limit > highmem) {
>> +			addr = memblock_alloc_range_nid(size, align, highmem,
>> +							*limit, nid, true);
>> +			*limit = highmem;
>> +		}
>> +	}
> 
> Not a big deal, but maybe better to do it in one function? 

Yes, same thought here.

-- 
Cheers,

David / dhildenb