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Date:   Mon, 19 Nov 2018 18:33:12 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Baoquan He <bhe@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        pifang@redhat.com, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, aarcange@redhat.com,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Hugh Dickins <hughd@google.com>
Subject: Re: Memory hotplug softlock issue
Message-ID: <20181119173312.GV22247@dhcp22.suse.cz>
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On Mon 19-11-18 15:10:16, Michal Hocko wrote:
[...]
> In other words. Why cannot we do the following?

Baoquan, this is certainly not the right fix but I would be really
curious whether it makes the problem go away.

> diff --git a/mm/migrate.c b/mm/migrate.c
> index f7e4bfdc13b7..7ccab29bcf9a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -324,19 +324,9 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>  		goto out;
>  
>  	page = migration_entry_to_page(entry);
> -
> -	/*
> -	 * Once page cache replacement of page migration started, page_count
> -	 * *must* be zero. And, we don't want to call wait_on_page_locked()
> -	 * against a page without get_page().
> -	 * So, we use get_page_unless_zero(), here. Even failed, page fault
> -	 * will occur again.
> -	 */
> -	if (!get_page_unless_zero(page))
> -		goto out;
>  	pte_unmap_unlock(ptep, ptl);
> -	wait_on_page_locked(page);
> -	put_page(page);
> +	page_lock(page);
> +	page_unlock(page);
>  	return;
>  out:
>  	pte_unmap_unlock(ptep, ptl);

-- 
Michal Hocko
SUSE Labs