linux-kernel - Re: [PATCH 3/3] dma/pool: Avoid allocating redundant pools

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Message-ID: <8d0c7b1c-fc12-4498-acbb-eaf6cab9ef3f@arm.com>
Date: Tue, 13 Jan 2026 16:14:44 +0000
From: Robin Murphy <robin.murphy@....com>
To: Baoquan He <bhe@...hat.com>
Cc: m.szyprowski@...sung.com, akpm@...ux-foundation.org, vbabka@...e.cz,
 david@...nel.org, iommu@...ts.linux-foundation.org, linux-mm@...ck.org,
 vladimir.kondratiev@...ileye.com, s-adivi@...com,
 linux-kernel@...r.kernel.org, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, rppt@...nel.org, surenb@...gle.com,
 mhocko@...e.com, jackmanb@...gle.com, hannes@...xchg.org, ziy@...dia.com
Subject: Re: [PATCH 3/3] dma/pool: Avoid allocating redundant pools

On 2026-01-13 10:16 am, Baoquan He wrote:
> On 01/12/26 at 03:46pm, Robin Murphy wrote:
>> On smaller systems, e.g. embedded arm64, it is common for all memory
>> to end up in ZONE_DMA32 or even ZONE_DMA. In such cases it is redundant
> 
> This is true and the whole series looks great to me. Do we need adjust
> warn_alloc() to handle empty DMA32 zone too like empty DMA zone case?

Hmm, I'd be inclined to think that if nobody's complaining already then 
we can probably just leave it as-is. A GFP_DMA32 allocation won't OOM 
unless *both* ZONE_DMA32 and ZONE_DMA are empty, right? At that point 
I'd imagine it's a bit more significant if someone who wants DMA32 
memory can't have any - don't we have a mechanism for reserving some 
"low" memory for kdump for pretty much this exact reason?

A special case for when ZONE_DMA is tiny such that GFP_DMA can be 
expected to fail often seems fair, but in general I'd expect that if 
GFP_DMA32 starts failing then it's more a sign of a genuine mismatch 
between the kernel's expectations and the system configuration.

Thanks,
Robin.

>> to allocate a nominal pool for an empty higher zone that just ends up
>> coming from a lower zone that should already have its own pool anyway.
>> We already have logic to skip allocating a ZONE_DMA pool when that is
>> empty, so generalise that to save memory in the case of other zones too.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>>   kernel/dma/pool.c | 19 ++++++++++++++-----
>>   1 file changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
>> index 2645cfb5718b..c5da29ad010c 100644
>> --- a/kernel/dma/pool.c
>> +++ b/kernel/dma/pool.c
>> @@ -184,6 +184,12 @@ static __init struct gen_pool *__dma_atomic_pool_init(size_t pool_size,
>>   	return pool;
>>   }
>>   
>> +#ifdef CONFIG_ZONE_DMA32
>> +#define has_managed_dma32 has_managed_zone(ZONE_DMA32)
>> +#else
>> +#define has_managed_dma32 false
>> +#endif
>> +
>>   static int __init dma_atomic_pool_init(void)
>>   {
>>   	int ret = 0;
>> @@ -199,17 +205,20 @@ static int __init dma_atomic_pool_init(void)
>>   	}
>>   	INIT_WORK(&atomic_pool_work, atomic_pool_work_fn);
>>   
>> -	atomic_pool_kernel = __dma_atomic_pool_init(atomic_pool_size,
>> +	/* All memory might be in the DMA zone(s) to begin with */
>> +	if (has_managed_zone(ZONE_NORMAL)) {
>> +		atomic_pool_kernel = __dma_atomic_pool_init(atomic_pool_size,
>>   						    GFP_KERNEL);
>> -	if (!atomic_pool_kernel)
>> -		ret = -ENOMEM;
>> +		if (!atomic_pool_kernel)
>> +			ret = -ENOMEM;
>> +	}
>>   	if (has_managed_dma()) {
>>   		atomic_pool_dma = __dma_atomic_pool_init(atomic_pool_size,
>>   						GFP_KERNEL | GFP_DMA);
>>   		if (!atomic_pool_dma)
>>   			ret = -ENOMEM;
>>   	}
>> -	if (IS_ENABLED(CONFIG_ZONE_DMA32)) {
>> +	if (has_managed_dma32) {
>>   		atomic_pool_dma32 = __dma_atomic_pool_init(atomic_pool_size,
>>   						GFP_KERNEL | GFP_DMA32);
>>   		if (!atomic_pool_dma32)
>> @@ -228,7 +237,7 @@ static inline struct gen_pool *dma_guess_pool(struct gen_pool *prev, gfp_t gfp)
>>   			return atomic_pool_dma ?: atomic_pool_dma32 ?: atomic_pool_kernel;
>>   		if (gfp & GFP_DMA32)
>>   			return atomic_pool_dma32 ?: atomic_pool_dma ?: atomic_pool_kernel;
>> -		return atomic_pool_kernel;
>> +		return atomic_pool_kernel ?: atomic_pool_dma32 ?: atomic_pool_dma;
>>   	}
>>   	if (prev == atomic_pool_kernel)
>>   		return atomic_pool_dma32 ? atomic_pool_dma32 : atomic_pool_dma;
>> -- 
>> 2.34.1
>>
>>
>