linux-kernel - Re: [PATCH] mm: fix panic in __alloc

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Message-ID: <20211102135201.GA4348@linux>
Date:   Tue, 2 Nov 2021 14:52:01 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Alexey Makhalov <amakhalov@...are.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Oscar Salvador <OSalvador@...e.com>
Subject: Re: [PATCH] mm: fix panic in __alloc_pages

On Tue, Nov 02, 2021 at 02:25:03PM +0100, Michal Hocko wrote:
> I think we want to learn how exactly Alexey brought that cpu up. Because
> his initial thought on add_cpu resp cpu_up doesn't seem to be correct.
> Or I am just not following the code properly. Once we know all those
> details we can get in touch with cpu hotplug maintainers and see what
> can we do.

I am not really familiar with CPU hot-onlining, but I have been taking a look.
As with memory, there are two different stages, hot-adding and onlining (and the
counterparts).

Part of the hot-adding being:

acpi_processor_get_info
 acpi_processor_hotadd_init
  arch_register_cpu
   register_cpu

One of the things that register_cpu() does is to set cpu->dev.bus pointing to
&cpu_subsys, which is:

struct bus_type cpu_subsys = {
	.name = "cpu",
	.dev_name = "cpu",
	.match = cpu_subsys_match,
#ifdef CONFIG_HOTPLUG_CPU
	.online = cpu_subsys_online,
	.offline = cpu_subsys_offline,
#endif
};

Then, the onlining part (in case of a udev rule or someone onlining the device)
would be:

online_store
 device_online
  cpu_subsys_online
   cpu_device_up
    cpu_up
     ...
     online node

Since Alexey disabled the udev rule and no one onlined the CPU, online_store()->
device_online() wasn't really called.

The following only applies to x86_64:
I think we got confused because cpu_device_up() is also called from add_cpu(),
but that is an exported function and x86 does not call add_cpu() unless for
debugging purposes (check kernel/torture.c and arch/x86/kernel/topology.c).
It does the onlining through online_store()...
So we can take add_cpu() off the equation here.

-- 
Oscar Salvador
SUSE Labs