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

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Message-Id: <8ab8d8a620dee0109f33f5cb63d6bfeed35aac37.1768230104.git.robin.murphy@arm.com>
Date: Mon, 12 Jan 2026 15:46:38 +0000
From: Robin Murphy <robin.murphy@....com>
To: m.szyprowski@...sung.com,
	akpm@...ux-foundation.org,
	vbabka@...e.cz,
	david@...nel.org
Cc: bhe@...hat.com,
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	vladimir.kondratiev@...ileye.com,
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	linux-kernel@...r.kernel.org,
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Subject: [PATCH 3/3] dma/pool: Avoid allocating redundant pools

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
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