Re: [PATCH v3] mm: page_alloc: dump migrate-failed pages

[Minchan Kim <minchan@kernel.org>]

On Wed, Mar 10, 2021 at 12:56:09PM -0800, Andrew Morton wrote:
> 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?

Oops. Now fixed.

> 
> > --- 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.

Something like this?

#if defined(CONFIG_DYNAMIC_DEBUG) || \
        (defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
/*
 *      dyndbg_dir="/sys/kernel/debug/dynamic_debug"
 * To enable:
 *      echo "func dump_migrate_failure_pages +p" > $dyndbg_dir/control
 * To disable:
 *      echo "func dump_migrate_failure_pages =_" > $dyndbg_dir/control
 */