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

From: Baoquan He <bhe@redhat.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: k-hagio-ab@nec.com, "lijiang@redhat.com" <lijiang@redhat.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	Dave Chinner <david@fromorbit.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>
Subject: Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree
Date: Fri, 8 Sep 2023 19:38:42 +0800	[thread overview]
Message-ID: <ZPsHwrjG+/6r7ln9@MiWiFi-R3L-srv> (raw)
In-Reply-To: <ZPsEqcREYksx9Ot8@pc636>

On 09/08/23 at 01:25pm, Uladzislau Rezki wrote:
> 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?

Maybe take it together with this patchset in v3. Thanks.