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Subject: Re: [PATCH v3] mm: page_alloc: dump migrate-failed pages
To:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        John Dias <joaodias@google.com>,
        Michal Hocko <mhocko@suse.com>, Jason Baron <jbaron@akamai.com>
References: <20210310180104.517886-1-minchan@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <cdca0d9b-0c7e-bafc-6d86-bc75654b16ae@redhat.com>
Date:   Thu, 11 Mar 2021 17:40:35 +0100
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On 10.03.21 19:01, Minchan Kim 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
> 
> Detail goes Documentation/admin-guide/dynamic-debug-howto.rst
> 
> A concern is utility functions in dump_page uses inconsistent
> loglevels.
> 
> __dump_page: KERN_WARNING
> __dump_page_owner: KERN_ALERT
>          stack_trace_print: KERN_DEFAULT
> 
> There are bunch of places to use the inconsistent loglevel
> utility functions(e.g., just grep dump_page/strace_trace_print).
> It's unfortunate but here we are. It could be addressed
> different patchset.
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> * from v2 - https://lore.kernel.org/linux-mm/20210308202047.1903802-1-minchan@kernel.org/
>    * remove ratelimit - mhocko
> 
> * from v1 - https://lore.kernel.org/linux-mm/20210217163603.429062-1-minchan@kernel.org/
>    * use dynamic debugging with system wide instead of per-call site - mhocko
> 
>   mm/page_alloc.c | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3e4b29ee2b1e..f42f3e208794 100644
> --- 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");

Not sure if we want that here. You can deterministically crash a kernel 
with panic_on_warn set - which is what some distros already do.

Some other way to dump the current call stack would be handy ...

> +		list_for_each_entry(page, page_list, lru)
> +			dump_page(page, "migration failure");
> +	}
> +}
> +#else
> +static inline void alloc_contig_dump_pages(struct list_head *page_list)
> +{
> +}
> +#endif
> +
>   /* [start, end) must belong to a single zone. */
>   static int __alloc_contig_migrate_range(struct compact_control *cc,
>   					unsigned long start, unsigned long end)
> @@ -8496,6 +8517,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>   				NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
>   	}
>   	if (ret < 0) {
> +		alloc_contig_dump_pages(&cc->migratepages);
>   		putback_movable_pages(&cc->migratepages);
>   		return ret;
>   	}
> 

For me, this is good enough for now.

-- 
Thanks,

David / dhildenb