Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

[Michael Ellerman]

Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
>> On Tue 10-10-17 23:05:08, Michael Ellerman wrote:
>>> Michal Hocko <mhocko@xxxxxxxxxx> writes:
>>> > From: Michal Hocko <mhocko@xxxxxxxx>
>>> > Memory offlining can fail just too eagerly under a heavy memory pressure.
>>> >
>>> > [ 5410.336792] page:ffffea22a646bd00 count:255 mapcount:252 mapping:ffff88ff926c9f38 index:0x3
>>> > [ 5410.336809] flags: 0x9855fe40010048(uptodate|active|mappedtodisk)
>>> > [ 5410.336811] page dumped because: isolation failed
>>> > [ 5410.336813] page->mem_cgroup:ffff8801cd662000
>>> > [ 5420.655030] memory offlining [mem 0x18b580000000-0x18b5ffffffff] failed
>>> >
>>> > Isolation has failed here because the page is not on LRU. Most probably
>>> > because it was on the pcp LRU cache or it has been removed from the LRU
>>> > already but it hasn't been freed yet. In both cases the page doesn't look
>>> > non-migrable so retrying more makes sense.
>>> 
>>> This breaks offline for me.
>>> 
>>> Prior to this commit:
>>>   /sys/devices/system/memory/memory0# time echo 0 > online
>>>   -bash: echo: write error: Device or resource busy
>>>   
>>>   real	0m0.001s
>>>   user	0m0.000s
>>>   sys	0m0.001s
>>> 
>>> After:
>>>   /sys/devices/system/memory/memory0# time echo 0 > online
>>>   -bash: echo: write error: Device or resource busy
>>>   
>>>   real	2m0.009s
>>>   user	0m0.000s
>>>   sys	1m25.035s
>>> 
>>> There's no way that block can be removed, it contains the kernel text,
>>> so it should instantly fail - which it used to.
>>
>> OK, that means that start_isolate_page_range should have failed but it
>> hasn't for some reason. I strongly suspect has_unmovable_pages is doing
>> something wrong. Is the kernel text marked somehow? E.g. PageReserved?
>
> I'm not sure how the text is marked, will have to dig into that.

Yeah it's reserved:

  $ grep __init_begin /proc/kallsyms
  c000000000d70000 T __init_begin
  $ ./page-types -r -a 0x0,0xd7
               flags	page-count       MB  symbolic-flags			long-symbolic-flags
  0x0000000100000000	       215       13  __________________________r_______________	reserved
               total	       215       13


I added some printks, we're getting EBUSY from do_migrate_range(pfn, end_pfn).

So we seem to just have an infinite loop:

  repeat:
  	/* start memory hot removal */
  	ret = -EINTR;
  	if (signal_pending(current))
  		goto failed_removal;
  
  	cond_resched();
  	lru_add_drain_all_cpuslocked();
  	drain_all_pages(zone);
  
  	pfn = scan_movable_pages(start_pfn, end_pfn);
  	if (pfn) { /* We have movable pages */
  		ret = do_migrate_range(pfn, end_pfn);
  		printk_ratelimited("memory-hotplug: migrate range returned %ld\n", ret);
  		goto repeat;
  	}


eg:

  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  __offline_pages: 354031 callbacks suppressed
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  memory-hotplug: migrate range returned -16
  __offline_pages: 355794 callbacks suppressed


cheers