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Date:   Mon, 23 Nov 2020 15:13:54 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Charan Teja Reddy <charante@codeaurora.org>
Cc:     akpm@linux-foundation.org, david@redhat.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: memory_hotplug: put migration failure information
 under DEBUG_VM
Message-ID: <20201123141354.GQ27488@dhcp22.suse.cz>
References: <1606140196-6053-1-git-send-email-charante@codeaurora.org>
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On Mon 23-11-20 19:33:16, Charan Teja Reddy wrote:
> When the pages are failed to get isolate or migrate, the page owner
> information along with page info is dumped. If there are continuous
> failures in migration(say page is pinned) or isolation, the log buffer
> is simply getting flooded with the page owner information. As most of
> the times page info is sufficient to know the causes for failures of
> migration or isolation, place the page owner information under DEBUG_VM.

I do not see why this path is any different from others that call
dump_page. Page owner can add a very valuable information to debug
the underlying reasons for failures here. It is an opt-in debugging
feature which needs to be enabled explicitly. So I would argue users
are ready to accept a lot of data in the kernel log.
 
> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
> ---
>  mm/memory_hotplug.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 63b2e46..f48f30d 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1326,7 +1326,10 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  
>  		} else {
>  			pr_warn("failed to isolate pfn %lx\n", pfn);
> -			dump_page(page, "isolation failed");
> +			__dump_page(page, "isolation failed");
> +#if defined(CONFIG_DEBUG_VM)
> +			dump_page_owner(page);
> +#endif
>  		}
>  		put_page(page);
>  	}
> @@ -1357,7 +1360,10 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  			list_for_each_entry(page, &source, lru) {
>  				pr_warn("migrating pfn %lx failed ret:%d ",
>  				       page_to_pfn(page), ret);
> -				dump_page(page, "migration failure");
> +				__dump_page(page, "migration failure");
> +#if defined(CONFIG_DEBUG_VM)
> +				dump_page_owner(page);
> +#endif
>  			}
>  			putback_movable_pages(&source);
>  		}
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
> member of the Code Aurora Forum, hosted by The Linux Foundation

-- 
Michal Hocko
SUSE Labs