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@arm.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;
--
2.34.1On 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@arm.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
>
>
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@arm.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
>>
>>
>
On 01/13/26 at 04:14pm, Robin Murphy wrote: > 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. Makes sense to me. I remember someone plans to take off both DMA and DMA32 zone in linux kernel, but that might not happen soon.
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