[PATCH v2 0/3] arm64: support page mapping percpu first chunk allocator

STALE1824d

Revision v2 of 4 in this series.

13 messages, 3 authors, 2021-08-05 · open the first message on its own page

[PATCH v2 0/3] arm64: support page mapping percpu first chunk allocator

From: Kefeng Wang <hidden>
Date: 2021-07-20 02:52:45

Percpu embedded first chunk allocator is the firstly option, but it
could fails on ARM64, eg,
  "percpu: max_distance=0x5fcfdc640000 too large for vmalloc space 0x781fefff0000"
  "percpu: max_distance=0x600000540000 too large for vmalloc space 0x7dffb7ff0000"
  "percpu: max_distance=0x5fff9adb0000 too large for vmalloc space 0x5dffb7ff0000"

then we could meet "WARNING: CPU: 15 PID: 461 at vmalloc.c:3087 pcpu_get_vm_areas+0x488/0x838",
even the system could not boot successfully.

Let's implement page mapping percpu first chunk allocator as a fallback
to the embedding allocator to increase the robustness of the system.

Also fix a crash when both NEED_PER_CPU_PAGE_FIRST_CHUNK and KASAN_VMALLOC enabled.

Tested on ARM64 qemu with cmdline "percpu_alloc=page" based on v5.14-rc2.

V2:
- fix build error when CONFIG_KASAN disabled, found by lkp@intel.com
- drop wrong __weak comment from kasan_populate_early_vm_area_shadow(),
  found by Marco Elver [off-list ref]

Kefeng Wang (3):
  vmalloc: Choose a better start address in vm_area_register_early()
  arm64: Support page mapping percpu first chunk allocator
  kasan: arm64: Fix pcpu_page_first_chunk crash with KASAN_VMALLOC

 arch/arm64/Kconfig         |  4 ++
 arch/arm64/mm/kasan_init.c | 17 ++++++++
 drivers/base/arch_numa.c   | 82 +++++++++++++++++++++++++++++++++-----
 include/linux/kasan.h      |  6 +++
 mm/kasan/init.c            |  5 +++
 mm/vmalloc.c               |  9 +++--
 6 files changed, 110 insertions(+), 13 deletions(-)

-- 
2.26.2


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[PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register_early()

From: Kefeng Wang <hidden>
Date: 2021-07-20 02:52:08

There are some fixed locations in the vmalloc area be reserved
in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
pcpu_page_first_chunk(), it calls vm_area_register_early() and
choose VMALLOC_START as the start address of vmap area which
could be conflicted with above address, then could trigger a
BUG_ON in vm_area_add_early().

Let's choose the end of existing address range in vmlist as the
start address instead of VMALLOC_START to avoid the BUG_ON.

Signed-off-by: Kefeng Wang <redacted>
---
 mm/vmalloc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d5cd52805149..a98cf97f032f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
  */
 void __init vm_area_register_early(struct vm_struct *vm, size_t align)
 {
-	static size_t vm_init_off __initdata;
+	unsigned long vm_start = VMALLOC_START;
+	struct vm_struct *tmp;
 	unsigned long addr;
 
-	addr = ALIGN(VMALLOC_START + vm_init_off, align);
-	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
+	for (tmp = vmlist; tmp; tmp = tmp->next)
+		vm_start = (unsigned long)tmp->addr + tmp->size;
 
+	addr = ALIGN(vm_start, align);
 	vm->addr = (void *)addr;
 
 	vm_area_add_early(vm);

-- 
2.26.2


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[PATCH v2 2/3] arm64: Support page mapping percpu first chunk allocator

From: Kefeng Wang <hidden>
Date: 2021-07-20 02:53:26

Percpu embedded first chunk allocator is the firstly option, but it
could fails on ARM64, eg,
  "percpu: max_distance=0x5fcfdc640000 too large for vmalloc space 0x781fefff0000"
  "percpu: max_distance=0x600000540000 too large for vmalloc space 0x7dffb7ff0000"
  "percpu: max_distance=0x5fff9adb0000 too large for vmalloc space 0x5dffb7ff0000"

then we could meet "WARNING: CPU: 15 PID: 461 at vmalloc.c:3087 pcpu_get_vm_areas+0x488/0x838",
even the system could not boot successfully.

Let's implement page mapping percpu first chunk allocator as a fallback
to the embedding allocator to increase the robustness of the system.

Signed-off-by: Kefeng Wang <redacted>
---
 arch/arm64/Kconfig       |  4 ++
 drivers/base/arch_numa.c | 82 +++++++++++++++++++++++++++++++++++-----
 2 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b5b13a932561..eacb5873ded1 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -1045,6 +1045,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
 	def_bool y
 	depends on NUMA
 
+config NEED_PER_CPU_PAGE_FIRST_CHUNK
+	def_bool y
+	depends on NUMA
+
 source "kernel/Kconfig.hz"
 
 config ARCH_SPARSEMEM_ENABLE

diff --git a/drivers/base/arch_numa.c b/drivers/base/arch_numa.c
index 4cc4e117727d..563b2013b75a 100644
--- a/drivers/base/arch_numa.c
+++ b/drivers/base/arch_numa.c

@@ -14,6 +14,7 @@
 #include <linux/of.h>
 
 #include <asm/sections.h>
+#include <asm/pgalloc.h>
 
 struct pglist_data *node_data[MAX_NUMNODES] __read_mostly;
 EXPORT_SYMBOL(node_data);

@@ -168,22 +169,83 @@ static void __init pcpu_fc_free(void *ptr, size_t size)
 	memblock_free_early(__pa(ptr), size);
 }
 
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+static void __init pcpu_populate_pte(unsigned long addr)
+{
+	pgd_t *pgd = pgd_offset_k(addr);
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	p4d = p4d_offset(pgd, addr);
+	if (p4d_none(*p4d)) {
+		pud_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		p4d_populate(&init_mm, p4d, new);
+	}
+
+	pud = pud_offset(p4d, addr);
+	if (pud_none(*pud)) {
+		pmd_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		pud_populate(&init_mm, pud, new);
+	}
+
+	pmd = pmd_offset(pud, addr);
+	if (!pmd_present(*pmd)) {
+		pte_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		pmd_populate_kernel(&init_mm, pmd, new);
+	}
+
+	return;
+
+err_alloc:
+	panic("%s: Failed to allocate %lu bytes align=%lx from=%lx\n",
+	      __func__, PAGE_SIZE, PAGE_SIZE, PAGE_SIZE);
+}
+#endif
+
 void __init setup_per_cpu_areas(void)
 {
 	unsigned long delta;
 	unsigned int cpu;
-	int rc;
+	int rc = -EINVAL;
+
+	if (pcpu_chosen_fc != PCPU_FC_PAGE) {
+		/*
+		 * Always reserve area for module percpu variables.  That's
+		 * what the legacy allocator did.
+		 */
+		rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
+					    PERCPU_DYNAMIC_RESERVE, PAGE_SIZE,
+					    pcpu_cpu_distance,
+					    pcpu_fc_alloc, pcpu_fc_free);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+		if (rc < 0)
+			pr_warn("PERCPU: %s allocator failed (%d), falling back to page size\n",
+				   pcpu_fc_names[pcpu_chosen_fc], rc);
+#endif
+	}
 
-	/*
-	 * Always reserve area for module percpu variables.  That's
-	 * what the legacy allocator did.
-	 */
-	rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
-				    PERCPU_DYNAMIC_RESERVE, PAGE_SIZE,
-				    pcpu_cpu_distance,
-				    pcpu_fc_alloc, pcpu_fc_free);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+	if (rc < 0)
+		rc = pcpu_page_first_chunk(PERCPU_MODULE_RESERVE,
+					   pcpu_fc_alloc,
+					   pcpu_fc_free,
+					   pcpu_populate_pte);
+#endif
 	if (rc < 0)
-		panic("Failed to initialize percpu areas.");
+		panic("Failed to initialize percpu areas (err=%d).", rc);
 
 	delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
 	for_each_possible_cpu(cpu)

-- 
2.26.2


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[PATCH v2 3/3] kasan: arm64: Fix pcpu_page_first_chunk crash with KASAN_VMALLOC

From: Kefeng Wang <hidden>
Date: 2021-07-20 02:54:24

With KASAN_VMALLOC and NEED_PER_CPU_PAGE_FIRST_CHUNK, it crashs,

Unable to handle kernel paging request at virtual address ffff7000028f2000
...
swapper pgtable: 64k pages, 48-bit VAs, pgdp=0000000042440000
[ffff7000028f2000] pgd=000000063e7c0003, p4d=000000063e7c0003, pud=000000063e7c0003, pmd=000000063e7b0003, pte=0000000000000000
Internal error: Oops: 96000007 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 5.13.0-rc4-00003-gc6e6e28f3f30-dirty #62
Hardware name: linux,dummy-virt (DT)
pstate: 200000c5 (nzCv daIF -PAN -UAO -TCO BTYPE=--)
pc : kasan_check_range+0x90/0x1a0
lr : memcpy+0x88/0xf4
sp : ffff80001378fe20
...
Call trace:
 kasan_check_range+0x90/0x1a0
 pcpu_page_first_chunk+0x3f0/0x568
 setup_per_cpu_areas+0xb8/0x184
 start_kernel+0x8c/0x328

The vm area used in vm_area_register_early() has no kasan shadow memory,
Let's add a new kasan_populate_early_vm_area_shadow() function to populate
the vm area shadow memory to fix the issue.

Signed-off-by: Kefeng Wang <redacted>
---
 arch/arm64/mm/kasan_init.c | 17 +++++++++++++++++
 include/linux/kasan.h      |  6 ++++++
 mm/kasan/init.c            |  5 +++++
 mm/vmalloc.c               |  1 +
 4 files changed, 29 insertions(+)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 61b52a92b8b6..46c1b3722901 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c

@@ -287,6 +287,23 @@ static void __init kasan_init_depth(void)
 	init_task.kasan_depth = 0;
 }
 
+#ifdef CONFIG_KASAN_VMALLOC
+void __init kasan_populate_early_vm_area_shadow(void *start, unsigned long size)
+{
+	unsigned long shadow_start, shadow_end;
+
+	if (!is_vmalloc_or_module_addr(start))
+		return;
+
+	shadow_start = (unsigned long)kasan_mem_to_shadow(start);
+	shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
+	shadow_end = (unsigned long)kasan_mem_to_shadow(start + size);
+	shadow_end = ALIGN(shadow_end, PAGE_SIZE);
+	kasan_map_populate(shadow_start, shadow_end,
+			   early_pfn_to_nid(virt_to_pfn(start)));
+}
+#endif
+
 void __init kasan_init(void)
 {
 	kasan_init_shadow();

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index dd874a1ee862..3f8c26d9ef82 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h

@@ -133,6 +133,8 @@ struct kasan_cache {
 	bool is_kmalloc;
 };
 
+void kasan_populate_early_vm_area_shadow(void *start, unsigned long size);
+
 slab_flags_t __kasan_never_merge(void);
 static __always_inline slab_flags_t kasan_never_merge(void)
 {

@@ -303,6 +305,10 @@ void kasan_restore_multi_shot(bool enabled);
 
 #else /* CONFIG_KASAN */
 
+static inline void kasan_populate_early_vm_area_shadow(void *start,
+						       unsigned long size)
+{ }
+
 static inline slab_flags_t kasan_never_merge(void)
 {
 	return 0;

diff --git a/mm/kasan/init.c b/mm/kasan/init.c
index cc64ed6858c6..d39577d088a1 100644
--- a/mm/kasan/init.c
+++ b/mm/kasan/init.c

@@ -279,6 +279,11 @@ int __ref kasan_populate_early_shadow(const void *shadow_start,
 	return 0;
 }
 
+void __init __weak kasan_populate_early_vm_area_shadow(void *start,
+						       unsigned long size)
+{
+}
+
 static void kasan_free_pte(pte_t *pte_start, pmd_t *pmd)
 {
 	pte_t *pte;

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a98cf97f032f..f19e07314ee5 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2249,6 +2249,7 @@ void __init vm_area_register_early(struct vm_struct *vm, size_t align)
 	vm->addr = (void *)addr;
 
 	vm_area_add_early(vm);
+	kasan_populate_early_vm_area_shadow(vm->addr, vm->size);
 }
 
 static void vmap_init_free_space(void)

-- 
2.26.2


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Re: [PATCH v2 3/3] kasan: arm64: Fix pcpu_page_first_chunk crash with KASAN_VMALLOC

From: Marco Elver <elver@google.com>
Date: 2021-07-22 11:01:11

On Tue, Jul 20, 2021 at 10:51AM +0800, Kefeng Wang wrote:

With KASAN_VMALLOC and NEED_PER_CPU_PAGE_FIRST_CHUNK, it crashs,

Unable to handle kernel paging request at virtual address ffff7000028f2000
...
swapper pgtable: 64k pages, 48-bit VAs, pgdp=0000000042440000
[ffff7000028f2000] pgd=000000063e7c0003, p4d=000000063e7c0003, pud=000000063e7c0003, pmd=000000063e7b0003, pte=0000000000000000
Internal error: Oops: 96000007 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 5.13.0-rc4-00003-gc6e6e28f3f30-dirty #62
Hardware name: linux,dummy-virt (DT)
pstate: 200000c5 (nzCv daIF -PAN -UAO -TCO BTYPE=--)
pc : kasan_check_range+0x90/0x1a0
lr : memcpy+0x88/0xf4
sp : ffff80001378fe20
...
Call trace:
 kasan_check_range+0x90/0x1a0
 pcpu_page_first_chunk+0x3f0/0x568
 setup_per_cpu_areas+0xb8/0x184
 start_kernel+0x8c/0x328

The vm area used in vm_area_register_early() has no kasan shadow memory,
Let's add a new kasan_populate_early_vm_area_shadow() function to populate
the vm area shadow memory to fix the issue.

Signed-off-by: Kefeng Wang <redacted>

Acked-by: Marco Elver <elver@google.com>

for the kasan bits.

quoted hunk

---
 arch/arm64/mm/kasan_init.c | 17 +++++++++++++++++
 include/linux/kasan.h      |  6 ++++++
 mm/kasan/init.c            |  5 +++++
 mm/vmalloc.c               |  1 +
 4 files changed, 29 insertions(+)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 61b52a92b8b6..46c1b3722901 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c

@@ -287,6 +287,23 @@ static void __init kasan_init_depth(void)
 	init_task.kasan_depth = 0;
 }
 
+#ifdef CONFIG_KASAN_VMALLOC
+void __init kasan_populate_early_vm_area_shadow(void *start, unsigned long size)
+{
+	unsigned long shadow_start, shadow_end;
+
+	if (!is_vmalloc_or_module_addr(start))
+		return;
+
+	shadow_start = (unsigned long)kasan_mem_to_shadow(start);
+	shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
+	shadow_end = (unsigned long)kasan_mem_to_shadow(start + size);
+	shadow_end = ALIGN(shadow_end, PAGE_SIZE);
+	kasan_map_populate(shadow_start, shadow_end,
+			   early_pfn_to_nid(virt_to_pfn(start)));
+}
+#endif
+
 void __init kasan_init(void)
 {
 	kasan_init_shadow();

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index dd874a1ee862..3f8c26d9ef82 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h

@@ -133,6 +133,8 @@ struct kasan_cache {
 	bool is_kmalloc;
 };
 
+void kasan_populate_early_vm_area_shadow(void *start, unsigned long size);
+
 slab_flags_t __kasan_never_merge(void);
 static __always_inline slab_flags_t kasan_never_merge(void)
 {

@@ -303,6 +305,10 @@ void kasan_restore_multi_shot(bool enabled);
 
 #else /* CONFIG_KASAN */
 
+static inline void kasan_populate_early_vm_area_shadow(void *start,
+						       unsigned long size)
+{ }
+
 static inline slab_flags_t kasan_never_merge(void)
 {
 	return 0;

diff --git a/mm/kasan/init.c b/mm/kasan/init.c
index cc64ed6858c6..d39577d088a1 100644
--- a/mm/kasan/init.c
+++ b/mm/kasan/init.c

@@ -279,6 +279,11 @@ int __ref kasan_populate_early_shadow(const void *shadow_start,
 	return 0;
 }
 
+void __init __weak kasan_populate_early_vm_area_shadow(void *start,
+						       unsigned long size)
+{
+}
+
 static void kasan_free_pte(pte_t *pte_start, pmd_t *pmd)
 {
 	pte_t *pte;

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a98cf97f032f..f19e07314ee5 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2249,6 +2249,7 @@ void __init vm_area_register_early(struct vm_struct *vm, size_t align)
 	vm->addr = (void *)addr;
 
 	vm_area_add_early(vm);
+	kasan_populate_early_vm_area_shadow(vm->addr, vm->size);
 }
 
 static void vmap_init_free_space(void)

-- 
2.26.2

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Re: [PATCH v2 3/3] kasan: arm64: Fix pcpu_page_first_chunk crash with KASAN_VMALLOC

From: Kefeng Wang <hidden>
Date: 2021-07-22 12:14:39

On 2021/7/22 19:00, Marco Elver wrote:

On Tue, Jul 20, 2021 at 10:51AM +0800, Kefeng Wang wrote:

quoted

With KASAN_VMALLOC and NEED_PER_CPU_PAGE_FIRST_CHUNK, it crashs,

Unable to handle kernel paging request at virtual address ffff7000028f2000
...
swapper pgtable: 64k pages, 48-bit VAs, pgdp=0000000042440000
[ffff7000028f2000] pgd=000000063e7c0003, p4d=000000063e7c0003, pud=000000063e7c0003, pmd=000000063e7b0003, pte=0000000000000000
Internal error: Oops: 96000007 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 5.13.0-rc4-00003-gc6e6e28f3f30-dirty #62
Hardware name: linux,dummy-virt (DT)
pstate: 200000c5 (nzCv daIF -PAN -UAO -TCO BTYPE=--)
pc : kasan_check_range+0x90/0x1a0
lr : memcpy+0x88/0xf4
sp : ffff80001378fe20
...
Call trace:
  kasan_check_range+0x90/0x1a0
  pcpu_page_first_chunk+0x3f0/0x568
  setup_per_cpu_areas+0xb8/0x184
  start_kernel+0x8c/0x328

The vm area used in vm_area_register_early() has no kasan shadow memory,
Let's add a new kasan_populate_early_vm_area_shadow() function to populate
the vm area shadow memory to fix the issue.

Signed-off-by: Kefeng Wang <redacted>

Acked-by: Marco Elver <elver@google.com>

for the kasan bits.

Thanks Marco.

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Re: [PATCH v2 0/3] arm64: support page mapping percpu first chunk allocator

From: Kefeng Wang <hidden>
Date: 2021-07-26 01:19:17

Hi Will and Catalin，any comments，kindly ping, thanks.

On 2021/7/20 10:51, Kefeng Wang wrote:

Percpu embedded first chunk allocator is the firstly option, but it
could fails on ARM64, eg,
   "percpu: max_distance=0x5fcfdc640000 too large for vmalloc space 0x781fefff0000"
   "percpu: max_distance=0x600000540000 too large for vmalloc space 0x7dffb7ff0000"
   "percpu: max_distance=0x5fff9adb0000 too large for vmalloc space 0x5dffb7ff0000"

then we could meet "WARNING: CPU: 15 PID: 461 at vmalloc.c:3087 pcpu_get_vm_areas+0x488/0x838",
even the system could not boot successfully.

Let's implement page mapping percpu first chunk allocator as a fallback
to the embedding allocator to increase the robustness of the system.

Also fix a crash when both NEED_PER_CPU_PAGE_FIRST_CHUNK and KASAN_VMALLOC enabled.

Tested on ARM64 qemu with cmdline "percpu_alloc=page" based on v5.14-rc2.

V2:
- fix build error when CONFIG_KASAN disabled, found by lkp@intel.com
- drop wrong __weak comment from kasan_populate_early_vm_area_shadow(),
   found by Marco Elver [off-list ref]

Kefeng Wang (3):
   vmalloc: Choose a better start address in vm_area_register_early()
   arm64: Support page mapping percpu first chunk allocator
   kasan: arm64: Fix pcpu_page_first_chunk crash with KASAN_VMALLOC

  arch/arm64/Kconfig         |  4 ++
  arch/arm64/mm/kasan_init.c | 17 ++++++++
  drivers/base/arch_numa.c   | 82 +++++++++++++++++++++++++++++++++-----
  include/linux/kasan.h      |  6 +++
  mm/kasan/init.c            |  5 +++
  mm/vmalloc.c               |  9 +++--
  6 files changed, 110 insertions(+), 13 deletions(-)

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Re: [PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register_early()

From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-08-01 15:23:19

On Tue, Jul 20, 2021 at 10:51:03AM +0800, Kefeng Wang wrote:

quoted hunk

There are some fixed locations in the vmalloc area be reserved
in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
pcpu_page_first_chunk(), it calls vm_area_register_early() and
choose VMALLOC_START as the start address of vmap area which
could be conflicted with above address, then could trigger a
BUG_ON in vm_area_add_early().

Let's choose the end of existing address range in vmlist as the
start address instead of VMALLOC_START to avoid the BUG_ON.

Signed-off-by: Kefeng Wang <redacted>
---
 mm/vmalloc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d5cd52805149..a98cf97f032f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
  */
 void __init vm_area_register_early(struct vm_struct *vm, size_t align)
 {
-	static size_t vm_init_off __initdata;
+	unsigned long vm_start = VMALLOC_START;
+	struct vm_struct *tmp;
 	unsigned long addr;
 
-	addr = ALIGN(VMALLOC_START + vm_init_off, align);
-	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
+	for (tmp = vmlist; tmp; tmp = tmp->next)
+		vm_start = (unsigned long)tmp->addr + tmp->size;
 
+	addr = ALIGN(vm_start, align);
 	vm->addr = (void *)addr;
 
 	vm_area_add_early(vm);

Is there a risk of breaking other architectures? It doesn't look like to
me but I thought I'd ask.

Also, instead of always picking the end, could we search for a range
that fits?

-- 
Catalin

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Re: [PATCH v2 2/3] arm64: Support page mapping percpu first chunk allocator

From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-08-01 16:01:23

On Tue, Jul 20, 2021 at 10:51:04AM +0800, Kefeng Wang wrote:

Percpu embedded first chunk allocator is the firstly option, but it
could fails on ARM64, eg,
  "percpu: max_distance=0x5fcfdc640000 too large for vmalloc space 0x781fefff0000"
  "percpu: max_distance=0x600000540000 too large for vmalloc space 0x7dffb7ff0000"
  "percpu: max_distance=0x5fff9adb0000 too large for vmalloc space 0x5dffb7ff0000"

then we could meet "WARNING: CPU: 15 PID: 461 at vmalloc.c:3087 pcpu_get_vm_areas+0x488/0x838",
even the system could not boot successfully.

Let's implement page mapping percpu first chunk allocator as a fallback
to the embedding allocator to increase the robustness of the system.

It looks like x86, powerpc and sparc implement their own
setup_per_cpu_areas(). I had a quick look on finding some commonalities
but I think it's a lot more hassle to make a generic version out of them
(powerpc looks the simplest though). I think we could add a generic
variant with the arm64 support and later migrate other architectures to
it if possible.

The patch looks ok to me otherwise but I'd need an ack from Greg as it
touches drivers/.

BTW, do we need something similar for the non-NUMA
setup_per_cpu_areas()? I can see this patch only enables
NEED_PER_CPU_PAGE_FIRST_CHUNK if NUMA.

Leaving the rest of the patch below for Greg.

quoted hunk

Signed-off-by: Kefeng Wang <redacted>
---
 arch/arm64/Kconfig       |  4 ++
 drivers/base/arch_numa.c | 82 +++++++++++++++++++++++++++++++++++-----
 2 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b5b13a932561..eacb5873ded1 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -1045,6 +1045,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
 	def_bool y
 	depends on NUMA
 
+config NEED_PER_CPU_PAGE_FIRST_CHUNK
+	def_bool y
+	depends on NUMA
+
 source "kernel/Kconfig.hz"
 
 config ARCH_SPARSEMEM_ENABLE

diff --git a/drivers/base/arch_numa.c b/drivers/base/arch_numa.c
index 4cc4e117727d..563b2013b75a 100644
--- a/drivers/base/arch_numa.c
+++ b/drivers/base/arch_numa.c

@@ -14,6 +14,7 @@
 #include <linux/of.h>
 
 #include <asm/sections.h>
+#include <asm/pgalloc.h>
 
 struct pglist_data *node_data[MAX_NUMNODES] __read_mostly;
 EXPORT_SYMBOL(node_data);

@@ -168,22 +169,83 @@ static void __init pcpu_fc_free(void *ptr, size_t size)
 	memblock_free_early(__pa(ptr), size);
 }
 
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+static void __init pcpu_populate_pte(unsigned long addr)
+{
+	pgd_t *pgd = pgd_offset_k(addr);
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	p4d = p4d_offset(pgd, addr);
+	if (p4d_none(*p4d)) {
+		pud_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		p4d_populate(&init_mm, p4d, new);
+	}
+
+	pud = pud_offset(p4d, addr);
+	if (pud_none(*pud)) {
+		pmd_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		pud_populate(&init_mm, pud, new);
+	}
+
+	pmd = pmd_offset(pud, addr);
+	if (!pmd_present(*pmd)) {
+		pte_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		pmd_populate_kernel(&init_mm, pmd, new);
+	}
+
+	return;
+
+err_alloc:
+	panic("%s: Failed to allocate %lu bytes align=%lx from=%lx\n",
+	      __func__, PAGE_SIZE, PAGE_SIZE, PAGE_SIZE);
+}
+#endif
+
 void __init setup_per_cpu_areas(void)
 {
 	unsigned long delta;
 	unsigned int cpu;
-	int rc;
+	int rc = -EINVAL;
+
+	if (pcpu_chosen_fc != PCPU_FC_PAGE) {
+		/*
+		 * Always reserve area for module percpu variables.  That's
+		 * what the legacy allocator did.
+		 */
+		rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
+					    PERCPU_DYNAMIC_RESERVE, PAGE_SIZE,
+					    pcpu_cpu_distance,
+					    pcpu_fc_alloc, pcpu_fc_free);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+		if (rc < 0)
+			pr_warn("PERCPU: %s allocator failed (%d), falling back to page size\n",
+				   pcpu_fc_names[pcpu_chosen_fc], rc);
+#endif
+	}
 
-	/*
-	 * Always reserve area for module percpu variables.  That's
-	 * what the legacy allocator did.
-	 */
-	rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
-				    PERCPU_DYNAMIC_RESERVE, PAGE_SIZE,
-				    pcpu_cpu_distance,
-				    pcpu_fc_alloc, pcpu_fc_free);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+	if (rc < 0)
+		rc = pcpu_page_first_chunk(PERCPU_MODULE_RESERVE,
+					   pcpu_fc_alloc,
+					   pcpu_fc_free,
+					   pcpu_populate_pte);
+#endif
 	if (rc < 0)
-		panic("Failed to initialize percpu areas.");
+		panic("Failed to initialize percpu areas (err=%d).", rc);
 
 	delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
 	for_each_possible_cpu(cpu)

-- 
2.26.2

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Re: [PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register_early()

From: Kefeng Wang <hidden>
Date: 2021-08-02 02:39:09

On 2021/8/1 23:23, Catalin Marinas wrote:

On Tue, Jul 20, 2021 at 10:51:03AM +0800, Kefeng Wang wrote:

quoted

There are some fixed locations in the vmalloc area be reserved
in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
pcpu_page_first_chunk(), it calls vm_area_register_early() and
choose VMALLOC_START as the start address of vmap area which
could be conflicted with above address, then could trigger a
BUG_ON in vm_area_add_early().

Let's choose the end of existing address range in vmlist as the
start address instead of VMALLOC_START to avoid the BUG_ON.

Signed-off-by: Kefeng Wang <redacted>
---
  mm/vmalloc.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d5cd52805149..a98cf97f032f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
   */
  void __init vm_area_register_early(struct vm_struct *vm, size_t align)
  {
-	static size_t vm_init_off __initdata;
+	unsigned long vm_start = VMALLOC_START;
+	struct vm_struct *tmp;
  	unsigned long addr;
  
-	addr = ALIGN(VMALLOC_START + vm_init_off, align);
-	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
+	for (tmp = vmlist; tmp; tmp = tmp->next)
+		vm_start = (unsigned long)tmp->addr + tmp->size;
  
+	addr = ALIGN(vm_start, align);
  	vm->addr = (void *)addr;
  
  	vm_area_add_early(vm);

Is there a risk of breaking other architectures? It doesn't look like to
me but I thought I'd ask.

Before this patch, vm_init_off is to record the offset from VMALLOC_START,

but it use VMALLOC_START as start address on the function 
vm_area_register_early()

called firstly,  this will cause the BUG_ON.

With this patch, the most important change is that we choose the start 
address via

dynamic calculate the 'start' address by traversing the list.

[wkf@localhost linux-next]$ git grep vm_area_register_early
arch/alpha/mm/init.c: vm_area_register_early(&console_remap_vm, PAGE_SIZE);
arch/x86/xen/p2m.c:     vm_area_register_early(&vm, PMD_SIZE * 
PMDS_PER_MID_PAGE);
mm/percpu.c:    vm_area_register_early(&vm, PAGE_SIZE);
[wkf@localhost linux-next]$ git grep vm_area_add_early
arch/arm/mm/ioremap.c:  vm_area_add_early(vm);
arch/arm64/mm/mmu.c:    vm_area_add_early(vma);

x86/alpha won't call vm_area_add_early(), only arm64 could call both vm_area_add_early()
and  vm_area_register_early() when this patchset is merged. so it won't break other architectures.

Also, instead of always picking the end, could we search for a range
that fits?

We only need a space in vmalloc range,  using end or a range in the 
middle is not different.

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Re: [PATCH v2 2/3] arm64: Support page mapping percpu first chunk allocator

From: Kefeng Wang <hidden>
Date: 2021-08-02 02:47:12

On 2021/8/1 23:53, Catalin Marinas wrote:

On Tue, Jul 20, 2021 at 10:51:04AM +0800, Kefeng Wang wrote:

quoted

Percpu embedded first chunk allocator is the firstly option, but it
could fails on ARM64, eg,
   "percpu: max_distance=0x5fcfdc640000 too large for vmalloc space 0x781fefff0000"
   "percpu: max_distance=0x600000540000 too large for vmalloc space 0x7dffb7ff0000"
   "percpu: max_distance=0x5fff9adb0000 too large for vmalloc space 0x5dffb7ff0000"

then we could meet "WARNING: CPU: 15 PID: 461 at vmalloc.c:3087 pcpu_get_vm_areas+0x488/0x838",
even the system could not boot successfully.

Let's implement page mapping percpu first chunk allocator as a fallback
to the embedding allocator to increase the robustness of the system.

It looks like x86, powerpc and sparc implement their own
setup_per_cpu_areas(). I had a quick look on finding some commonalities
but I think it's a lot more hassle to make a generic version out of them
(powerpc looks the simplest though). I think we could add a generic
variant with the arm64 support and later migrate other architectures to
it if possible.

Ok, let's do it later, I could try to make some cleanup after the 
patchset is merged ;)

The patch looks ok to me otherwise but I'd need an ack from Greg as it
touches drivers/.

the arch_numa is only used ARM64 and riscv, the 
NEED_PER_CPU_PAGE_FIRST_CHUNK

is not enabled on RISCV, so it's no bad effect.

BTW, do we need something similar for the non-NUMA
setup_per_cpu_areas()? I can see this patch only enables
NEED_PER_CPU_PAGE_FIRST_CHUNK if NUMA.

Leaving the rest of the patch below for Greg.

quoted

Signed-off-by: Kefeng Wang <redacted>
---
  arch/arm64/Kconfig       |  4 ++
  drivers/base/arch_numa.c | 82 +++++++++++++++++++++++++++++++++++-----
  2 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b5b13a932561..eacb5873ded1 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -1045,6 +1045,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
  	def_bool y
  	depends on NUMA
  
+config NEED_PER_CPU_PAGE_FIRST_CHUNK
+	def_bool y
+	depends on NUMA
+
  source "kernel/Kconfig.hz"
  
  config ARCH_SPARSEMEM_ENABLE

diff --git a/drivers/base/arch_numa.c b/drivers/base/arch_numa.c
index 4cc4e117727d..563b2013b75a 100644
--- a/drivers/base/arch_numa.c
+++ b/drivers/base/arch_numa.c

@@ -14,6 +14,7 @@
  #include <linux/of.h>
  
  #include <asm/sections.h>
+#include <asm/pgalloc.h>
  
  struct pglist_data *node_data[MAX_NUMNODES] __read_mostly;
  EXPORT_SYMBOL(node_data);

@@ -168,22 +169,83 @@ static void __init pcpu_fc_free(void *ptr, size_t size)
  	memblock_free_early(__pa(ptr), size);
  }
  
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+static void __init pcpu_populate_pte(unsigned long addr)
+{
+	pgd_t *pgd = pgd_offset_k(addr);
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	p4d = p4d_offset(pgd, addr);
+	if (p4d_none(*p4d)) {
+		pud_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		p4d_populate(&init_mm, p4d, new);
+	}
+
+	pud = pud_offset(p4d, addr);
+	if (pud_none(*pud)) {
+		pmd_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		pud_populate(&init_mm, pud, new);
+	}
+
+	pmd = pmd_offset(pud, addr);
+	if (!pmd_present(*pmd)) {
+		pte_t *new;
+
+		new = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+		if (!new)
+			goto err_alloc;
+		pmd_populate_kernel(&init_mm, pmd, new);
+	}
+
+	return;
+
+err_alloc:
+	panic("%s: Failed to allocate %lu bytes align=%lx from=%lx\n",
+	      __func__, PAGE_SIZE, PAGE_SIZE, PAGE_SIZE);
+}
+#endif
+
  void __init setup_per_cpu_areas(void)
  {
  	unsigned long delta;
  	unsigned int cpu;
-	int rc;
+	int rc = -EINVAL;
+
+	if (pcpu_chosen_fc != PCPU_FC_PAGE) {
+		/*
+		 * Always reserve area for module percpu variables.  That's
+		 * what the legacy allocator did.
+		 */
+		rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
+					    PERCPU_DYNAMIC_RESERVE, PAGE_SIZE,
+					    pcpu_cpu_distance,
+					    pcpu_fc_alloc, pcpu_fc_free);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+		if (rc < 0)
+			pr_warn("PERCPU: %s allocator failed (%d), falling back to page size\n",
+				   pcpu_fc_names[pcpu_chosen_fc], rc);
+#endif
+	}
  
-	/*
-	 * Always reserve area for module percpu variables.  That's
-	 * what the legacy allocator did.
-	 */
-	rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
-				    PERCPU_DYNAMIC_RESERVE, PAGE_SIZE,
-				    pcpu_cpu_distance,
-				    pcpu_fc_alloc, pcpu_fc_free);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+	if (rc < 0)
+		rc = pcpu_page_first_chunk(PERCPU_MODULE_RESERVE,
+					   pcpu_fc_alloc,
+					   pcpu_fc_free,
+					   pcpu_populate_pte);
+#endif
  	if (rc < 0)
-		panic("Failed to initialize percpu areas.");
+		panic("Failed to initialize percpu areas (err=%d).", rc);
  
  	delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
  	for_each_possible_cpu(cpu)

-- 
2.26.2

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Re: [PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register_early()

From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-08-04 11:14:22

On Mon, Aug 02, 2021 at 10:39:04AM +0800, Kefeng Wang wrote:

On 2021/8/1 23:23, Catalin Marinas wrote:

quoted

On Tue, Jul 20, 2021 at 10:51:03AM +0800, Kefeng Wang wrote:

quoted

There are some fixed locations in the vmalloc area be reserved
in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
pcpu_page_first_chunk(), it calls vm_area_register_early() and
choose VMALLOC_START as the start address of vmap area which
could be conflicted with above address, then could trigger a
BUG_ON in vm_area_add_early().

Let's choose the end of existing address range in vmlist as the
start address instead of VMALLOC_START to avoid the BUG_ON.

Signed-off-by: Kefeng Wang <redacted>
---
  mm/vmalloc.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d5cd52805149..a98cf97f032f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
   */
  void __init vm_area_register_early(struct vm_struct *vm, size_t align)
  {
-	static size_t vm_init_off __initdata;
+	unsigned long vm_start = VMALLOC_START;
+	struct vm_struct *tmp;
  	unsigned long addr;
-	addr = ALIGN(VMALLOC_START + vm_init_off, align);
-	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
+	for (tmp = vmlist; tmp; tmp = tmp->next)
+		vm_start = (unsigned long)tmp->addr + tmp->size;
+	addr = ALIGN(vm_start, align);
  	vm->addr = (void *)addr;
  	vm_area_add_early(vm);

Is there a risk of breaking other architectures? It doesn't look like to
me but I thought I'd ask.

Before this patch, vm_init_off is to record the offset from VMALLOC_START,

but it use VMALLOC_START as start address on the function
vm_area_register_early()

called firstly,  this will cause the BUG_ON.

With this patch, the most important change is that we choose the start
address via

dynamic calculate the 'start' address by traversing the list.

[wkf@localhost linux-next]$ git grep vm_area_register_early
arch/alpha/mm/init.c: vm_area_register_early(&console_remap_vm, PAGE_SIZE);
arch/x86/xen/p2m.c:     vm_area_register_early(&vm, PMD_SIZE *
PMDS_PER_MID_PAGE);
mm/percpu.c:    vm_area_register_early(&vm, PAGE_SIZE);
[wkf@localhost linux-next]$ git grep vm_area_add_early
arch/arm/mm/ioremap.c:  vm_area_add_early(vm);
arch/arm64/mm/mmu.c:    vm_area_add_early(vma);

x86/alpha won't call vm_area_add_early(), only arm64 could call both vm_area_add_early()
and  vm_area_register_early() when this patchset is merged. so it won't break other architectures.

Thanks for checking.

quoted

Also, instead of always picking the end, could we search for a range
that fits?

We only need a space in vmalloc range,  using end or a range in the middle
is not different.

I was thinking of making it more future-proof in case one registers a
vm area towards the end of the range. It's fairly easy to pick a range
in the middle now that you are adding a list traversal.

-- 
Catalin

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Re: [PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register_early()

From: Kefeng Wang <hidden>
Date: 2021-08-05 12:46:26

On 2021/8/4 19:14, Catalin Marinas wrote:

On Mon, Aug 02, 2021 at 10:39:04AM +0800, Kefeng Wang wrote:

quoted

On 2021/8/1 23:23, Catalin Marinas wrote:

quoted

On Tue, Jul 20, 2021 at 10:51:03AM +0800, Kefeng Wang wrote:

quoted

There are some fixed locations in the vmalloc area be reserved
in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
pcpu_page_first_chunk(), it calls vm_area_register_early() and
choose VMALLOC_START as the start address of vmap area which
could be conflicted with above address, then could trigger a
BUG_ON in vm_area_add_early().

Let's choose the end of existing address range in vmlist as the
start address instead of VMALLOC_START to avoid the BUG_ON.

Signed-off-by: Kefeng Wang <redacted>
---
   mm/vmalloc.c | 8 +++++---
   1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d5cd52805149..a98cf97f032f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
    */
   void __init vm_area_register_early(struct vm_struct *vm, size_t align)
   {
-	static size_t vm_init_off __initdata;
+	unsigned long vm_start = VMALLOC_START;
+	struct vm_struct *tmp;
   	unsigned long addr;
-	addr = ALIGN(VMALLOC_START + vm_init_off, align);
-	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
+	for (tmp = vmlist; tmp; tmp = tmp->next)
+		vm_start = (unsigned long)tmp->addr + tmp->size;
+	addr = ALIGN(vm_start, align);
   	vm->addr = (void *)addr;
   	vm_area_add_early(vm);

Is there a risk of breaking other architectures? It doesn't look like to
me but I thought I'd ask.

Before this patch, vm_init_off is to record the offset from VMALLOC_START,

but it use VMALLOC_START as start address on the function
vm_area_register_early()

called firstly,  this will cause the BUG_ON.

With this patch, the most important change is that we choose the start
address via

dynamic calculate the 'start' address by traversing the list.

[wkf@localhost linux-next]$ git grep vm_area_register_early
arch/alpha/mm/init.c: vm_area_register_early(&console_remap_vm, PAGE_SIZE);
arch/x86/xen/p2m.c:     vm_area_register_early(&vm, PMD_SIZE *
PMDS_PER_MID_PAGE);
mm/percpu.c:    vm_area_register_early(&vm, PAGE_SIZE);
[wkf@localhost linux-next]$ git grep vm_area_add_early
arch/arm/mm/ioremap.c:  vm_area_add_early(vm);
arch/arm64/mm/mmu.c:    vm_area_add_early(vma);

x86/alpha won't call vm_area_add_early(), only arm64 could call both vm_area_add_early()
and  vm_area_register_early() when this patchset is merged. so it won't break other architectures.

Thanks for checking.

quoted

Also, instead of always picking the end, could we search for a range
that fits?

We only need a space in vmalloc range,  using end or a range in the middle
is not different.

I was thinking of making it more future-proof in case one registers a
vm area towards the end of the range. It's fairly easy to pick a range
in the middle now that you are adding a list traversal.

ok,  will chose a suitable hole in the vmalloc range.

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