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Date:   Fri, 28 Apr 2023 13:07:28 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Yajun Deng <yajun.deng@linux.dev>
Cc:     david@redhat.com, osalvador@suse.de, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/memory_hotplug: fix dead loop in offline_pages()
Message-ID: <ZEuo8PiA3EzVnrEi@dhcp22.suse.cz>
References: <20230428100846.95535-1-yajun.deng@linux.dev>
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On Fri 28-04-23 18:08:46, Yajun Deng wrote:
> When migration failure in do_migrate_range() and then the
> scan_movable_pages() will most likely return the same movable pfn.
> In this case, there is no condition to bail out, they will
> always run like this:
> 
> ...
> [878020.623959] migrating pfn 1727813 failed ret:1
> [878020.623960] page:00000000faa9673c refcount:3 mapcount:0 mapping:00000000144ccd79 index:0x14280025 pfn:0x1727813
> [878020.623962] memcg:ffffa0ff82d5a000
> [878020.623962] aops:def_blk_aops ino:fd00001
> [878020.623964] flags: 0x17ffffc000206a(referenced|dirty|active|workingset|private|node=0|zone=2|lastcpupid=0x1fffff)
> [878020.623966] raw: 0017ffffc000206a ffffb0d14f50fbd8 ffffb0d14f50fbd8 ffffa0ff9c155018
> [878020.623967] raw: 0000000014280025 ffffa10327d702d8 00000003ffffffff ffffa0ff82d5a000
> [878020.623968] page dumped because: migration failure
> [878020.626196] migrating pfn 1727813 failed ret:1
> [878020.626198] page:00000000faa9673c refcount:3 mapcount:0 mapping:00000000144ccd79 index:0x14280025 pfn:0x1727813
> [878020.626200] memcg:ffffa0ff82d5a000
> [878020.626200] aops:def_blk_aops ino:fd00001
> [878020.626202] flags: 0x17ffffc000206a(referenced|dirty|active|workingset|private|node=0|zone=2|lastcpupid=0x1fffff)
> [878020.626204] raw: 0017ffffc000206a ffffb0d14f50fbd8 ffffb0d14f50fbd8 ffffa0ff9c155018
> [878020.626205] raw: 0000000014280025 ffffa10327d702d8 00000003ffffffff ffffa0ff82d5a000
> [878020.626206] page dumped because: migration failure
> ...
> 
> Bail out when migration failures reach 3 times.
> 
> Fixes: bb8965bd82fd ("mm, memory_hotplug: deobfuscate migration part of offlining")
> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>

Any hard coded failure retry limit will lead to premature failures. Have
a look at 72b39cfc4d75 ("mm, memory_hotplug: do not fail offlining too
early"). A proper way to deal with this is to implement termination from
the userspace (e.g. timeout $TIMEOUT /bin/echo 0 > $PATH_TO_MEM/online)
Nacked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memory_hotplug.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 8e0fa209d533..72dd385b8892 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1800,11 +1800,12 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  			struct zone *zone, struct memory_group *group)
>  {
>  	const unsigned long end_pfn = start_pfn + nr_pages;
> -	unsigned long pfn, system_ram_pages = 0;
> +	unsigned long pfn, tmp_pfn, system_ram_pages = 0;
>  	const int node = zone_to_nid(zone);
>  	unsigned long flags;
>  	struct memory_notify arg;
>  	char *reason;
> +	int count = 0;
>  	int ret;
>  
>  	/*
> @@ -1887,12 +1888,20 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  
>  			cond_resched();
>  
> +			tmp_pfn = pfn;
>  			ret = scan_movable_pages(pfn, end_pfn, &pfn);
>  			if (!ret) {
> -				/*
> -				 * TODO: fatal migration failures should bail
> -				 * out
> -				 */
> +				if (pfn == tmp_pfn)
> +					count++;
> +				else
> +					count = 0;
> +
> +				if (unlikely(count == 3)) {
> +					ret = -EBUSY;
> +					reason = "migration failure";
> +					goto failed_removal_isolated;
> +				}
> +
>  				do_migrate_range(pfn, end_pfn);
>  			}
>  		} while (!ret);
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs