Re: [PATCH v2 4/5] mm, memory-hotplug: Rework unregister_mem_sect_under_nodes

From: Oscar Salvador <osalvador@suse.de>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: akpm@linux-foundation.org, mhocko@suse.com,
	dan.j.williams@intel.com, pavel.tatashin@microsoft.com,
	jglisse@redhat.com, Jonathan.Cameron@huawei.com,
	rafael@kernel.org, david@redhat.com, linux-mm@kvack.org,
	Oscar Salvador <osalvador@suse.com>
Subject: Re: [PATCH v2 4/5] mm, memory-hotplug: Rework unregister_mem_sect_under_nodes
Date: Mon, 25 Mar 2019 08:40:32 +0100	[thread overview]
Message-ID: <20190325074027.vhybenecc6hk7kxs@d104.suse.de> (raw)
In-Reply-To: <45d6b6ed-ae84-f2d5-0d57-dc2e28938ce0@arm.com>

On Sun, Mar 24, 2019 at 12:18:26PM +0530, Anshuman Khandual wrote:
> Hello Oscar,

Hi Anshuman,

> Passing down node ID till unregister_mem_sect_under_nodes() solves the problem of
> querying struct page for nid but the current code assumes that the pfn range for
> any given memory section can have different node IDs. Hence it scans over the
> section and try to remove all possible node <---> memory block sysfs links.
> 
> I am just wondering is that assumption even correct ? Can we really have a memory
> section which belongs to different nodes ? Is that even possible.

Yes, current code assumes that, but looking at when we init sections at boot
stage, it seems like a 1:1 map to me.

E.g, in memory_present(), we do encode the nid in section's section_mem_map
field to use that later on in sparse_init(), and get the node we should allocate
the data structures from.

And in memory_present() itself, in case we do not use page's flags field,
we end up using the section_to_node_table[] table, which is clearly a 1:1 map.

So, I might be wrong here, but I think that we do not really have nodes mixed
in a section.

-- 
Oscar Salvador
SUSE L3