Re: [PATCH 1/6] mm: migration: Factor out code to compute expected number of page references

[Mel Gorman <mgorman@suse.de>]

On Tue, Dec 11, 2018 at 06:21:38PM +0100, Jan Kara wrote:
> Factor out function to compute number of expected page references in
> migrate_page_move_mapping(). Note that we move hpage_nr_pages() and
> page_has_private() checks from under xas_lock_irq() however this is safe
> since we hold page lock.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/migrate.c | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index f7e4bfdc13b7..789c7bc90a0c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -428,6 +428,22 @@ static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
>  }
>  #endif /* CONFIG_BLOCK */
>  
> +static int expected_page_refs(struct page *page)
> +{
> +	int expected_count = 1;
> +
> +	/*
> +	 * Device public or private pages have an extra refcount as they are
> +	 * ZONE_DEVICE pages.
> +	 */
> +	expected_count += is_device_private_page(page);
> +	expected_count += is_device_public_page(page);
> +	if (page->mapping)
> +		expected_count += hpage_nr_pages(page) + page_has_private(page);
> +
> +	return expected_count;
> +}
> +

I noticed during testing that THP allocation success rates under the
mmtests configuration global-dhp__workload_thpscale-madvhugepage-xfs were
terrible with massive latencies introduced somewhere in the series. I
haven't tried chasing it down as it's relatively late but this block
looked odd and I missed it the first time.

This page->mapping test is relevant for the "Anonymous page without
mapping" check but I think it's wrong. An anonymous page without mapping
doesn't have a NULL mapping, it sets PAGE_MAPPING_ANON and the field can
be special in other ways. I think you meant to use page_mapping(page)
here, not page->mapping?

-- 
Mel Gorman
SUSE Labs