RE: [PATCH V2 4/6] mm: rename the global section array to mem_sections

From: "HAGIO KAZUHITO(萩尾　一仁)" <k-hagio-ab@nec.com>
To: Baoquan He <bhe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Dong Aisheng <dongas86@gmail.com>,
	Dong Aisheng <aisheng.dong@nxp.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	open list <linux-kernel@vger.kernel.org>,
	Dave Young <dyoung@redhat.com>, Vivek Goyal <vgoyal@redhat.com>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>
Subject: RE: [PATCH V2 4/6] mm: rename the global section array to mem_sections
Date: Wed, 2 Jun 2021 05:02:31 +0000	[thread overview]
Message-ID: <TYYPR01MB67770ED42E65667E304F2FF9DD3D9@TYYPR01MB6777.jpnprd01.prod.outlook.com> (raw)
In-Reply-To: <20210602030353.GA409140@MiWiFi-R3L-srv>

-----Original Message-----
> On 06/02/21 at 01:11am, HAGIO KAZUHITO(萩尾 一仁) wrote:
> > -----Original Message-----
> > > On Tue, 1 Jun 2021 10:40:09 +0200 David Hildenbrand <david@redhat.com> wrote:
> > >
> > > > > Thanks, i explained the reason during my last reply.
> > > > > Andrew has already picked this patch to -mm tree.
> > > >
> > > > Just because it's in Andrews tree doesn't mean it will end up upstream. ;)
> > > >
> > > > Anyhow, no really strong opinion, it's simply unnecessary code churn
> > > > that makes bisecting harder without real value IMHO.
> > >
> > > I think it's a good change - mem_sections refers to multiple instances
> > > of a mem_section.  Churn is a pain, but that's the price we pay for more
> > > readable code.  And for having screwed it up originally ;)
> >
> > From a makedumpfile/crash-utility viewpoint, I don't deny kernel improvement
> > and probably the change will not be hard for them to support, but I'd like
> > you to remember that the tool users will need to update them for the change.
> 
> As VIM user, I can understand Aisheng's feeling on the mem_section
> variable which has the same symbol name as its type. Meanwhile it does
> cause makedumpfile/crash having to be changed accordingly.
> 
> Maybe we can carry it when any essential change is needed in both kernel
> and makedumpfile/crash around it.

Yes, that is a possible option.

> 
> >
> > The situation where we need to update the tools for new kernels is usual, but
> > there are not many cases that they cannot even start session, and this change
> 
> By the way, Kazu, about a case starting session, could you be more specific
> or rephrase? I may not get it clearly. Thanks.

As for the current crash, the "mem_section" symbol is used to determine
which memory model is used.

        if (kernel_symbol_exists("mem_section"))
                vt->flags |= SPARSEMEM;
        else if (kernel_symbol_exists("mem_map")) {
                get_symbol_data("mem_map", sizeof(char *), &vt->mem_map);
                vt->flags |= FLATMEM;
        } else
                vt->flags |= DISCONTIGMEM;

So without updating, crash will assume that the memory model is DISCONTIGMEM,
fail during vm_init() and cannot start a session.  This is an imitation of
the situation though:

-       if (kernel_symbol_exists("mem_section"))
+       if (kernel_symbol_exists("mem_sectionX"))

# crash
...
crash: invalid structure member offset: pglist_data_node_mem_map
           FILE: memory.c  LINE: 16420  FUNCTION: dump_memory_nodes()

[/root/bin/crash] error trace: 465304 => 4ac2bf => 4aae19 => 57f4d7

  57f4d7: OFFSET_verify+164
  4aae19: dump_memory_nodes+5321
  4ac2bf: vm_init+4031
  465304: main_loop+392

#

Every time a kernel is released, there are some changes that crash can
start up with but cannot run a specific crash's command, but a change
that crash cannot start up like this case does not occur often.

Also as for makedumpfile, the "SYMBOL(mem_section)" vmcore entry is used
to determine the memory model, so it will fail with the following error
without an update.

# ./makedumpfile --mem-usage /proc/kcore 
get_mem_map: Can't distinguish the memory type.

makedumpfile Failed.

Thanks,
Kazu