Re: [RFC] mm: Allow ZONE_DMA32 to be disabled via kernel command line
From: Robin Murphy
Date: Thu Jan 26 2023 - 14:15:42 EST
On 2023-01-26 16:43, Georgi Djakov wrote:
From: Chris Goldsworthy <quic_cgoldswo@xxxxxxxxxxx>
It's useful to have an option to disable the ZONE_DMA32 during boot as
CONFIG_ZONE_DMA32 is by default enabled (on multiplatform kernels for
example). There are platforms that do not use this zone and in some high
memory pressure scenarios this would help on easing kswapd (to leave file
backed memory intact / unreclaimed). When the ZONE_DMA32 is enabled on
these platforms - kswapd is woken up more easily and drains the file cache
which leads to some performance issues.
Signed-off-by: Chris Goldsworthy <quic_cgoldswo@xxxxxxxxxxx>
[georgi: updated commit text]
Signed-off-by: Georgi Djakov <quic_c_gdjako@xxxxxxxxxxx>
---
The main question here is whether we can have a kernel command line
option to disable CONFIG_ZONE_DMA32 during boot (at least on arm64).
I can imagine this being useful also for Linux distros.
FWIW I'd say that "disabled" and "left empty then awkwardly tiptoed
around in a few places" are very different notions...
However, I'm just going to take a step back and read the commit message
a few more times... Given what it claims, I can't help but ask why
wouldn't we want a parameter to control kswapd's behaviour and address
that issue directly, rather than a massive hammer that breaks everyone
allocating explicitly or implicitly with __GFP_DMA32 (especially on
systems where it doesn't normally matter because all memory is below 4GB
anyway), just to achieve one rather niche side-effect?
Thanks,
Robin.
.../admin-guide/kernel-parameters.txt | 5 +++++
arch/arm64/mm/init.c | 20 ++++++++++++++++-
arch/x86/mm/init.c | 20 ++++++++++++++++-
include/linux/dma-direct.h | 22 +++++++++++++++++++
kernel/dma/direct.c | 5 +++--
kernel/dma/pool.c | 8 +++----
6 files changed, 72 insertions(+), 8 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cb12df402161..854ff65ac6b0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1070,6 +1070,11 @@
Disable Dynamic DMA Window support. Use this
to workaround buggy firmware.
+ disable_dma32= [KNL]
+ Dynamically disable ZONE_DMA32 on kernels compiled with
+ CONFIG_ZONE_DMA32=y.
+
+
disable_ipv6= [IPV6]
See Documentation/networking/ipv6.rst.
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 58a0bb2c17f1..1a56098c0e19 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -118,6 +118,12 @@ static int __init reserve_crashkernel_low(unsigned long long low_size)
return 0;
}
+/*
+ * Provide a run-time mean of disabling ZONE_DMA32 if it is enabled via
+ * CONFIG_ZONE_DMA32.
+ */
+static bool disable_dma32 __ro_after_init;
+
/*
* reserve_crashkernel() - reserves memory for crash kernel
*
@@ -244,7 +250,7 @@ static void __init zone_sizes_init(void)
max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
#endif
#ifdef CONFIG_ZONE_DMA32
- max_zone_pfns[ZONE_DMA32] = PFN_DOWN(dma32_phys_limit);
+ max_zone_pfns[ZONE_DMA32] = disable_dma32 ? 0 : PFN_DOWN(dma32_phys_limit);
if (!arm64_dma_phys_limit)
arm64_dma_phys_limit = dma32_phys_limit;
#endif
@@ -253,6 +259,18 @@ static void __init zone_sizes_init(void)
free_area_init(max_zone_pfns);
}
+static int __init early_disable_dma32(char *buf)
+{
+ if (!buf)
+ return -EINVAL;
+
+ if (!strcmp(buf, "on"))
+ disable_dma32 = true;
+
+ return 0;
+}
+early_param("disable_dma32", early_disable_dma32);
+
int pfn_is_map_memory(unsigned long pfn)
{
phys_addr_t addr = PFN_PHYS(pfn);
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index cb258f58fdc8..b8af7e2f21f5 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -112,6 +112,12 @@ static unsigned long min_pfn_mapped;
static bool __initdata can_use_brk_pgt = true;
+/*
+ * Provide a run-time mean of disabling ZONE_DMA32 if it is enabled via
+ * CONFIG_ZONE_DMA32.
+ */
+static bool disable_dma32 __ro_after_init;
+
/*
* Pages returned are already directly mapped.
*
@@ -1032,7 +1038,7 @@ void __init zone_sizes_init(void)
max_zone_pfns[ZONE_DMA] = min(MAX_DMA_PFN, max_low_pfn);
#endif
#ifdef CONFIG_ZONE_DMA32
- max_zone_pfns[ZONE_DMA32] = min(MAX_DMA32_PFN, max_low_pfn);
+ max_zone_pfns[ZONE_DMA32] = disable_dma32 ? 0 : min(MAX_DMA32_PFN, max_low_pfn);
#endif
max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
#ifdef CONFIG_HIGHMEM
@@ -1042,6 +1048,18 @@ void __init zone_sizes_init(void)
free_area_init(max_zone_pfns);
}
+static int __init early_disable_dma32(char *buf)
+{
+ if (!buf)
+ return -EINVAL;
+
+ if (!strcmp(buf, "on"))
+ disable_dma32 = true;
+
+ return 0;
+}
+early_param("disable_dma32", early_disable_dma32);
+
__visible DEFINE_PER_CPU_ALIGNED(struct tlb_state, cpu_tlbstate) = {
.loaded_mm = &init_mm,
.next_asid = 1,
diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index 18aade195884..ed69618cf1fc 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -24,6 +24,28 @@ struct bus_dma_region {
u64 offset;
};
+static inline bool zone_dma32_is_empty(int node)
+{
+#ifdef CONFIG_ZONE_DMA32
+ pg_data_t *pgdat = NODE_DATA(node);
+
+ return zone_is_empty(&pgdat->node_zones[ZONE_DMA32]);
+#else
+ return true;
+#endif
+}
+
+static inline bool zone_dma32_are_empty(void)
+{
+ int node;
+
+ for_each_node(node)
+ if (!zone_dma32_is_empty(node))
+ return false;
+
+ return true;
+}
+
static inline dma_addr_t translate_phys_to_dma(struct device *dev,
phys_addr_t paddr)
{
diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 63859a101ed8..754210c65658 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -60,7 +60,7 @@ static gfp_t dma_direct_optimal_gfp_mask(struct device *dev, u64 dma_mask,
*phys_limit = dma_to_phys(dev, dma_limit);
if (*phys_limit <= DMA_BIT_MASK(zone_dma_bits))
return GFP_DMA;
- if (*phys_limit <= DMA_BIT_MASK(32))
+ if (*phys_limit <= DMA_BIT_MASK(32) && !zone_dma32_is_empty(dev_to_node(dev)))
return GFP_DMA32;
return 0;
}
@@ -145,7 +145,8 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
if (IS_ENABLED(CONFIG_ZONE_DMA32) &&
phys_limit < DMA_BIT_MASK(64) &&
- !(gfp & (GFP_DMA32 | GFP_DMA))) {
+ !(gfp & (GFP_DMA32 | GFP_DMA)) &&
+ !zone_dma32_is_empty(node)) {
gfp |= GFP_DMA32;
goto again;
}
diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
index 4d40dcce7604..8e79903fbda8 100644
--- a/kernel/dma/pool.c
+++ b/kernel/dma/pool.c
@@ -71,7 +71,7 @@ static bool cma_in_zone(gfp_t gfp)
end = cma_get_base(cma) + size - 1;
if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp & GFP_DMA))
return end <= DMA_BIT_MASK(zone_dma_bits);
- if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32))
+ if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32) && !zone_dma32_are_empty())
return end <= DMA_BIT_MASK(32);
return true;
}
@@ -153,7 +153,7 @@ static void atomic_pool_work_fn(struct work_struct *work)
if (IS_ENABLED(CONFIG_ZONE_DMA))
atomic_pool_resize(atomic_pool_dma,
GFP_KERNEL | GFP_DMA);
- if (IS_ENABLED(CONFIG_ZONE_DMA32))
+ if (IS_ENABLED(CONFIG_ZONE_DMA32) && !zone_dma32_are_empty())
atomic_pool_resize(atomic_pool_dma32,
GFP_KERNEL | GFP_DMA32);
atomic_pool_resize(atomic_pool_kernel, GFP_KERNEL);
@@ -209,7 +209,7 @@ static int __init dma_atomic_pool_init(void)
if (!atomic_pool_dma)
ret = -ENOMEM;
}
- if (IS_ENABLED(CONFIG_ZONE_DMA32)) {
+ if (IS_ENABLED(CONFIG_ZONE_DMA32) && !zone_dma32_are_empty()) {
atomic_pool_dma32 = __dma_atomic_pool_init(atomic_pool_size,
GFP_KERNEL | GFP_DMA32);
if (!atomic_pool_dma32)
@@ -224,7 +224,7 @@ postcore_initcall(dma_atomic_pool_init);
static inline struct gen_pool *dma_guess_pool(struct gen_pool *prev, gfp_t gfp)
{
if (prev == NULL) {
- if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32))
+ if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32) && !zone_dma32_are_empty())
return atomic_pool_dma32;
if (atomic_pool_dma && (gfp & GFP_DMA))
return atomic_pool_dma;