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

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Message-ID: <aWYbcnglF5P/WhL4@MiWiFi-R3L-srv>
Date: Tue, 13 Jan 2026 18:16:18 +0800
From: Baoquan He <bhe@...hat.com>
To: Robin Murphy <robin.murphy@....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 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?

> 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
> 
>