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

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Message-Id: <b8e8ffdf-4f7b-2a02-5869-53b23da645d0@linux.vnet.ibm.com>
Date:   Tue, 5 Sep 2017 11:59:36 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

On 09/04/2017 01:51 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> 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.
> 
> __offline_pages seems rather cluttered when it comes to the retry
> logic. We have 5 retries at maximum and a timeout. We could argue
> whether the timeout makes sense but failing just because of a race when
> somebody isoltes a page from LRU or puts it on a pcp LRU lists is just
> wrong. It only takes it to race with a process which unmaps some pages
> and remove them from the LRU list and we can fail the whole offline
> because of something that is a temporary condition and actually not
> harmful for the offline. Please note that unmovable pages should be
> already excluded during start_isolate_page_range.
> 
> Fix this by removing the max retry count and only rely on the timeout
> resp. interruption by a signal from the userspace. Also retry rather
> than fail when check_pages_isolated sees some !free pages because those
> could be a result of the race as well.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/memory_hotplug.c | 40 ++++++++++------------------------------
>  1 file changed, 10 insertions(+), 30 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 459bbc182d10..c9dcbe6d2ac6 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1597,7 +1597,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  {
>  	unsigned long pfn, nr_pages, expire;
>  	long offlined_pages;
> -	int ret, drain, retry_max, node;
> +	int ret, node;
>  	unsigned long flags;
>  	unsigned long valid_start, valid_end;
>  	struct zone *zone;
> @@ -1634,43 +1634,25 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  
>  	pfn = start_pfn;
>  	expire = jiffies + timeout;
> -	drain = 0;
> -	retry_max = 5;
>  repeat:
>  	/* start memory hot removal */
> -	ret = -EAGAIN;
> +	ret = -EBUSY;
>  	if (time_after(jiffies, expire))
>  		goto failed_removal;
>  	ret = -EINTR;
>  	if (signal_pending(current))
>  		goto failed_removal;
> -	ret = 0;
> -	if (drain) {
> -		lru_add_drain_all_cpuslocked();
> -		cond_resched();
> -		drain_all_pages(zone);
> -	}

Why we had this condition before that only when we fail in migration
later in do_migrate_range function, drain the lru lists in the next
attempt. Why not from the first attempt itself ? Just being curious.