Re: [PATCH 0/6] mm: make movable onlining suck less

[Michal Hocko]

On Wed 05-04-17 10:48:52, Reza Arbab wrote:
> On Wed, Apr 05, 2017 at 08:42:39AM +0200, Michal Hocko wrote:
> >On Tue 04-04-17 16:43:39, Reza Arbab wrote:
> >>Okay, getting further. With this I can again repeatedly add and remove,
> >>but now I'm seeing a weird variation of that earlier issue:
> >>
> >>1. add_memory(), online_movable
> >>  /sys/devices/system/node/nodeX/memoryY symlinks are created.
> >>
> >>2. offline, remove_memory()
> >>  The node is offlined, since all memory has been removed, so all of
> >>  /sys/devices/system/node/nodeX is gone. This is normal.
> >>
> >>3. add_memory(), online_movable
> >>  The node is onlined, so /sys/devices/system/node/nodeX is recreated,
> >>  and the memory is added, but just like earlier in this email thread,
> >>  the memoryY links are not there.
> >
> >Could you add some printks to see why the sysfs creation failed please?
> 
> Ah, simple enough. It's this, right at the top of
> register_mem_sect_under_node():
> 
> 	if (!node_online(nid))
> 		return 0;
> 
> That being the case, I really don't understand why your patches make any
> difference. Is node_set_online() being called later than before somehow?

This is really interesting. Because add_memory_resource does the
following
	/* call arch's memory hotadd */
	ret = arch_add_memory(nid, start, size);

	if (ret < 0)
		goto error;

	/* we online node here. we can't roll back from here. */
	node_set_online(nid);

so we are setting the node online _after_ arch_add_memory but the code
which adds those sysfs file is called from

arch_add_memory
  __add_pages
    __add_section
      register_new_memory
        register_mem_sect_under_node
          node_online check

I haven't touched this part. What is the point of this check anyway? We
have already associated all the pages with a node (and with a zone prior
to my patches) so we _know_ how to create those links. The check goes
back to the initial submissions. Gary is not available anymore so we
cannot ask. But I completely fail to see how my changes could have made
any difference.

I assume that things start working after you remove that check? Btw. if
you put printk to the original kernel does it see the node online? I
would be also interested whether you see try_offline_node setting the
node offline in the original code.

Thanks!
-- 
Michal Hocko
SUSE Labs