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Date:   Wed, 10 Mar 2021 12:56:09 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        John Dias <joaodias@google.com>,
        Michal Hocko <mhocko@suse.com>,
        David Hildenbrand <david@redhat.com>,
        Jason Baron <jbaron@akamai.com>
Subject: Re: [PATCH v3] mm: page_alloc: dump migrate-failed pages
Message-Id: <20210310125609.359888dc65562fbed4b1f088@linux-foundation.org>
In-Reply-To: <20210310180104.517886-1-minchan@kernel.org>
References: <20210310180104.517886-1-minchan@kernel.org>
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On Wed, 10 Mar 2021 10:01:04 -0800 Minchan Kim <minchan@kernel.org> wrote:

> Currently, debugging CMA allocation failures is quite limited.
> The most commong source of these failures seems to be page
> migration which doesn't provide any useful information on the
> reason of the failure by itself. alloc_contig_range can report
> those failures as it holds a list of migrate-failed pages.
> 
> page refcount, mapcount with page flags on dump_page are
> helpful information to deduce the culprit. Furthermore,
> dump_page_owner was super helpful to find long term pinner
> who initiated the page allocation.
> 
> The reason it approach with dynamic debug is the debug message
> could emit lots of noises as alloc_contig_range calls more
> frequently since it's a best effort allocator.
> 
> There are two ifdefery conditions to support common dyndbg options:
> 
> - CONFIG_DYNAMIC_DEBUG_CORE && DYNAMIC_DEBUG_MODULE
> It aims for supporting the feature with only specific file
> with adding ccflags.
> 
> - CONFIG_DYNAMIC_DEBUG
> It aims for supporting the feature with system wide globally.
> 
> A simple example to enable the feature:
> 
> Admin could enable the dump like this(by default, disabled)
> 
> 	echo "func dump_migrate_failure_pages +p" > control
> 
> Admin could disable it.
> 
> 	echo "func dump_migrate_failure_pages =_" > control

I think the changelog is out of sync.  Did you mean
"alloc_contig_dump_pages" here?

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8453,6 +8453,27 @@ static unsigned long pfn_max_align_up(unsigned long pfn)
>  				pageblock_nr_pages));
>  }

> +#if defined(CONFIG_DYNAMIC_DEBUG) || \
> +	(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
> +static void alloc_contig_dump_pages(struct list_head *page_list)
> +{
> +	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor,
> +			"migrate failure");
> +
> +	if (DYNAMIC_DEBUG_BRANCH(descriptor)) {
> +		struct page *page;
> +
> +		WARN(1, "failed callstack");
> +		list_for_each_entry(page, page_list, lru)
> +			dump_page(page, "migration failure");
> +	}
> +}

I doubt if everyone is familiar with dynamic debug.  It might be kind
to add a little comment over this, telling people how to turn it on and
off.