Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

[Baoquan He <bhe@redhat.com>]

On 09/08/23 at 01:51am, HAGIO KAZUHITO(萩尾 一仁) wrote:
> On 2023/09/07 18:58, Baoquan He wrote:
> > On 09/07/23 at 11:39am, Uladzislau Rezki wrote:
> >> On Thu, Sep 07, 2023 at 10:17:39AM +0800, Baoquan He wrote:
> >>> Add Kazu and Lianbo to CC, and kexec mailing list
> >>>
> >>> On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> >>>> Store allocated objects in a separate nodes. A va->va_start
> >>>> address is converted into a correct node where it should
> >>>> be placed and resided. An addr_to_node() function is used
> >>>> to do a proper address conversion to determine a node that
> >>>> contains a VA.
> >>>>
> >>>> Such approach balances VAs across nodes as a result an access
> >>>> becomes scalable. Number of nodes in a system depends on number
> >>>> of CPUs divided by two. The density factor in this case is 1/2.
> >>>>
> >>>> Please note:
> >>>>
> >>>> 1. As of now allocated VAs are bound to a node-0. It means the
> >>>>     patch does not give any difference comparing with a current
> >>>>     behavior;
> >>>>
> >>>> 2. The global vmap_area_lock, vmap_area_root are removed as there
> >>>>     is no need in it anymore. The vmap_area_list is still kept and
> >>>>     is _empty_. It is exported for a kexec only;
> >>>
> >>> I haven't taken a test, while accessing all nodes' busy tree to get
> >>> va of the lowest address could severely impact kcore reading efficiency
> >>> on system with many vmap nodes. People doing live debugging via
> >>> /proc/kcore will get a little surprise.
> >>>
> >>>
> >>> Empty vmap_area_list will break makedumpfile utility, Crash utility
> >>> could be impactd too. I checked makedumpfile code, it relys on
> >>> vmap_area_list to deduce the vmalloc_start value.
> >>>
> >> It is left part and i hope i fix it in v3. The problem here is
> >> we can not give an opportunity to access to vmap internals from
> >> outside. This is just not correct, i.e. you are not allowed to
> >> access the list directly.
> > 
> > Right. Thanks for the fix in v3, that is a relief of makedumpfile and
> > crash.
> > 
> > Hi Kazu,
> > 
> > Meanwhile, I am thinking if we should evaluate the necessity of
> > vmap_area_list in makedumpfile and Crash. In makedumpfile, we just use
> > vmap_area_list to deduce VMALLOC_START. Wondering if we can export
> > VMALLOC_START directly. Surely, the lowest va->va_start in vmap_area_list
> > is a tighter low boundary of vmalloc area and can reduce unnecessary
> > scanning below the lowest va. Not sure if this is the reason people
> > decided to export vmap_area_list.
> 
> The kernel commit acd99dbf5402 introduced the original vmlist entry to 
> vmcoreinfo, but there is no information about why it did not export 
> VMALLOC_START directly.
> 
> If VMALLOC_START is exported directly to vmcoreinfo, I think it would be 
> enough for makedumpfile.

Thanks for confirmation, Kazu.

Then, below draft patch should be enough to export VMALLOC_START
instead, and remove vmap_area_list. In order to get the base address of
vmalloc area, constructing a vmap_area_list from multiple busy-tree
seems not worth.

diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
index 599e8d3bcbc3..3cb1ea09ff26 100644
--- a/Documentation/admin-guide/kdump/vmcoreinfo.rst
+++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst
@@ -65,11 +65,11 @@ Defines the beginning of the text section. In general, _stext indicates
 the kernel start address. Used to convert a virtual address from the
 direct kernel map to a physical address.
 
-vmap_area_list
---------------
+VMALLOC_START
+-------------
 
-Stores the virtual area list. makedumpfile gets the vmalloc start value
-from this variable and its value is necessary for vmalloc translation.
+Stores the base address of vmalloc area. makedumpfile gets this value and
+its value is necessary for vmalloc translation.
 
 mem_map
 -------
diff --git a/arch/arm64/kernel/crash_core.c b/arch/arm64/kernel/crash_core.c
index 66cde752cd74..2a24199a9b81 100644
--- a/arch/arm64/kernel/crash_core.c
+++ b/arch/arm64/kernel/crash_core.c
@@ -23,7 +23,6 @@ void arch_crash_save_vmcoreinfo(void)
 	/* Please note VMCOREINFO_NUMBER() uses "%d", not "%x" */
 	vmcoreinfo_append_str("NUMBER(MODULES_VADDR)=0x%lx\n", MODULES_VADDR);
 	vmcoreinfo_append_str("NUMBER(MODULES_END)=0x%lx\n", MODULES_END);
-	vmcoreinfo_append_str("NUMBER(VMALLOC_START)=0x%lx\n", VMALLOC_START);
 	vmcoreinfo_append_str("NUMBER(VMALLOC_END)=0x%lx\n", VMALLOC_END);
 	vmcoreinfo_append_str("NUMBER(VMEMMAP_START)=0x%lx\n", VMEMMAP_START);
 	vmcoreinfo_append_str("NUMBER(VMEMMAP_END)=0x%lx\n", VMEMMAP_END);
diff --git a/arch/riscv/kernel/crash_core.c b/arch/riscv/kernel/crash_core.c
index 55f1d7856b54..5c39cedd2c5c 100644
--- a/arch/riscv/kernel/crash_core.c
+++ b/arch/riscv/kernel/crash_core.c
@@ -9,7 +9,6 @@ void arch_crash_save_vmcoreinfo(void)
 	VMCOREINFO_NUMBER(phys_ram_base);
 
 	vmcoreinfo_append_str("NUMBER(PAGE_OFFSET)=0x%lx\n", PAGE_OFFSET);
-	vmcoreinfo_append_str("NUMBER(VMALLOC_START)=0x%lx\n", VMALLOC_START);
 	vmcoreinfo_append_str("NUMBER(VMALLOC_END)=0x%lx\n", VMALLOC_END);
 	vmcoreinfo_append_str("NUMBER(VMEMMAP_START)=0x%lx\n", VMEMMAP_START);
 	vmcoreinfo_append_str("NUMBER(VMEMMAP_END)=0x%lx\n", VMEMMAP_END);
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index c720be70c8dd..91810b4e9510 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -253,7 +253,6 @@ extern long vread_iter(struct iov_iter *iter, const char *addr, size_t count);
 /*
  *	Internals.  Don't use..
  */
-extern struct list_head vmap_area_list;
 extern __init void vm_area_add_early(struct vm_struct *vm);
 extern __init void vm_area_register_early(struct vm_struct *vm, size_t align);
 
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 03a7932cde0a..91af87930770 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -617,7 +617,7 @@ static int __init crash_save_vmcoreinfo_init(void)
 	VMCOREINFO_SYMBOL_ARRAY(swapper_pg_dir);
 #endif
 	VMCOREINFO_SYMBOL(_stext);
-	VMCOREINFO_SYMBOL(vmap_area_list);
+	vmcoreinfo_append_str("NUMBER(VMALLOC_START)=0x%lx\n", VMALLOC_START);
 
 #ifndef CONFIG_NUMA
 	VMCOREINFO_SYMBOL(mem_map);
diff --git a/kernel/kallsyms_selftest.c b/kernel/kallsyms_selftest.c
index b4cac76ea5e9..8a689b4ff4f9 100644
--- a/kernel/kallsyms_selftest.c
+++ b/kernel/kallsyms_selftest.c
@@ -89,7 +89,6 @@ static struct test_item test_items[] = {
 	ITEM_DATA(kallsyms_test_var_data_static),
 	ITEM_DATA(kallsyms_test_var_bss),
 	ITEM_DATA(kallsyms_test_var_data),
-	ITEM_DATA(vmap_area_list),
 #endif
 };
 
diff --git a/mm/nommu.c b/mm/nommu.c
index 7f9e9e5a0e12..8c6686176ebd 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -131,8 +131,6 @@ int follow_pfn(struct vm_area_struct *vma, unsigned long address,
 }
 EXPORT_SYMBOL(follow_pfn);
 
-LIST_HEAD(vmap_area_list);
-
 void vfree(const void *addr)
 {
 	kfree(addr);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 50d8239b82df..0a02633a9566 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -729,8 +729,7 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
 
 
 static DEFINE_SPINLOCK(free_vmap_area_lock);
-/* Export for kexec only */
-LIST_HEAD(vmap_area_list);
+
 static bool vmap_initialized __read_mostly;
 
 /*
-- 
2.41.0