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

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Sep 08, 2023 at 02:44:56PM +0800, Baoquan He wrote:
> On 09/08/23 at 05:01am, HAGIO KAZUHITO(萩尾 一仁) wrote:
> > 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);
> 
> Right, they are useless now. I updated to remove them in below patch.
> 
> From a867fada34fd9e96528fcc5e72ae50b3b5685015 Mon Sep 17 00:00:00 2001
> From: Baoquan He <bhe@redhat.com>
> Date: Fri, 8 Sep 2023 11:53:22 +0800
> Subject: [PATCH] mm/vmalloc: remove vmap_area_list
> Content-type: text/plain
> 
> Earlier, vmap_area_list is exported to vmcoreinfo so that makedumpfile
> get the base address of vmalloc area. Now, vmap_area_list is empty, so
> export VMALLOC_START to vmcoreinfo instead, and remove vmap_area_list.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  Documentation/admin-guide/kdump/vmcoreinfo.rst | 8 ++++----
>  arch/arm64/kernel/crash_core.c                 | 1 -
>  arch/riscv/kernel/crash_core.c                 | 1 -
>  include/linux/vmalloc.h                        | 1 -
>  kernel/crash_core.c                            | 4 +---
>  kernel/kallsyms_selftest.c                     | 1 -
>  mm/nommu.c                                     | 2 --
>  mm/vmalloc.c                                   | 3 +--
>  8 files changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
> index 599e8d3bcbc3..c11bd4b1ceb1 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
> +since 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..a9faaf7e5f7d 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);
> @@ -658,8 +658,6 @@ static int __init crash_save_vmcoreinfo_init(void)
>  	VMCOREINFO_OFFSET(free_area, free_list);
>  	VMCOREINFO_OFFSET(list_head, next);
>  	VMCOREINFO_OFFSET(list_head, prev);
> -	VMCOREINFO_OFFSET(vmap_area, va_start);
> -	VMCOREINFO_OFFSET(vmap_area, list);
>  	VMCOREINFO_LENGTH(zone.free_area, MAX_ORDER + 1);
>  	log_buf_vmcoreinfo_setup();
>  	VMCOREINFO_LENGTH(free_area.free_list, MIGRATE_TYPES);
> 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
> 
Appreciate for your great input. This patch can go as standalone
with slight commit message updating or i can take it and send it
out as part of v3.

Either way i am totally fine. What do you prefer?

Thank you!

--
Uladzislau Rezki