Re: [PATCH 2/6] mm: add get_pageblock_migratetype_nolock() for cases where locking is undesirable

[Joonsoo Kim]

On Fri, Feb 28, 2014 at 03:15:00PM +0100, Vlastimil Babka wrote:
> In order to prevent race with set_pageblock_migratetype, most of calls to
> get_pageblock_migratetype have been moved under zone->lock. For the remaining
> call sites, the extra locking is undesirable, notably in free_hot_cold_page().
> 
> This patch introduces a _nolock version to be used on these call sites, where
> a wrong value does not affect correctness. The function makes sure that the
> value does not exceed valid migratetype numbers. Such too-high values are
> assumed to be a result of race and caller-supplied fallback value is returned
> instead.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  include/linux/mmzone.h | 24 ++++++++++++++++++++++++
>  mm/compaction.c        | 14 +++++++++++---
>  mm/memory-failure.c    |  3 ++-
>  mm/page_alloc.c        | 22 +++++++++++++++++-----
>  mm/vmstat.c            |  2 +-
>  5 files changed, 55 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index fac5509..7c3f678 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -75,6 +75,30 @@ enum {
>  
>  extern int page_group_by_mobility_disabled;
>  
> +/*
> + * When called without zone->lock held, a race with set_pageblock_migratetype
> + * may result in bogus values. Use this variant only when this does not affect
> + * correctness, and taking zone->lock would be costly. Values >= MIGRATE_TYPES
> + * are considered to be a result of this race and the value of race_fallback
> + * argument is returned instead.
> + */
> +static inline int get_pageblock_migratetype_nolock(struct page *page,
> +	int race_fallback)
> +{
> +	int ret = get_pageblock_flags_group(page, PB_migrate, PB_migrate_end);
> +
> +	if (unlikely(ret >= MIGRATE_TYPES))
> +		ret = race_fallback;
> +
> +	return ret;
> +}

How about below forms?

get_pageblock_migratetype_locked(struct page *page)
get_pageblock_migratetype(struct page *page, int race_fallback)

get_pageblock_migratetype() and _nolock looks error-prone because developer
who try to use get_pageblock_migratetype() may not know that it needs lock.

Thanks.
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