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

Baoquan He
2023-09-08T06:46:00Z

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