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

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Date:   Fri, 13 Oct 2017 22:42:46 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

Vlastimil Babka <vbabka@...e.cz> writes:
> On 10/11/2017 08:51 AM, Michal Hocko wrote:
>> On Wed 11-10-17 13:37:50, Michael Ellerman wrote:
>>> Michal Hocko <mhocko@...nel.org> writes:
>>>> On Tue 10-10-17 23:05:08, Michael Ellerman wrote:
>>>>> Michal Hocko <mhocko@...nel.org> writes:
>>>>>> From: Michal Hocko <mhocko@...e.com>
>>>>>>
>>>>>> Memory offlining can fail just too eagerly under a heavy memory pressure.
...
>>>>>
>>>>> 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
>
> Well, that means offline didn't actually work for that block even before
> this patch, right? Is it even a movable_node block? I guess not?

Correct. It should fail.

>>>>> 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.
>
> Ah, right. So your complain is really about that the failure is not
> instant anymore for blocks that can't be offlined.

Yes. Previously it failed instantly, now it doesn't fail, and loops
infinitely (once the 2 minute limit is removed).

>> This is really strange! As you write in other email the page is
>> reserved. That means that some of the earlier checks 
>> 	if (zone_idx(zone) == ZONE_MOVABLE)
>> 		return false;
>> 	mt = get_pageblock_migratetype(page);
>> 	if (mt == MIGRATE_MOVABLE || is_migrate_cma(mt))
>
> The MIGRATE_MOVABLE check is indeed bogus, because that doesn't
> guarantee there are no unmovable pages in the block (CMA block OTOH
> should be a guarantee).

OK I'll try that and get back to you.

cheers


>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 3badcedf96a7..5b4d85ae445c 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7355,9 +7355,6 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>>  	 */
>>  	if (zone_idx(zone) == ZONE_MOVABLE)
>>  		return false;
>> -	mt = get_pageblock_migratetype(page);
>> -	if (mt == MIGRATE_MOVABLE || is_migrate_cma(mt))
>> -		return false;
>>  
>>  	pfn = page_to_pfn(page);
>>  	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
>> @@ -7368,6 +7365,9 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>>  
>>  		page = pfn_to_page(check);
>>  
>> +		if (PageReserved(page))
>> +			return true;
>> +
>>  		/*
>>  		 * Hugepages are not in LRU lists, but they're movable.
>>  		 * We need not scan over tail pages bacause we don't
>>