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

["HAGIO KAZUHITO(萩尾 一仁)" <k-hagio-ab@nec.com>]

On 2023/09/08 13:43, Baoquan He wrote:
> 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. 

also the following entries can be removed.

         VMCOREINFO_OFFSET(vmap_area, va_start);
         VMCOREINFO_OFFSET(vmap_area, list);

Thanks,
Kazu

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;
>   
>   /*