Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner

David Rientjes
2014-06-05T00:02:18Z

On Wed, 4 Jun 2014, Vlastimil Babka wrote:

> diff --git a/mm/compaction.c b/mm/compaction.c
> index ae7db5f..3dce5a7 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -640,11 +640,18 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> }
>
> /*
> - * Skip if free. page_order cannot be used without zone->lock
> - * as nothing prevents parallel allocations or buddy merging.
> + * Skip if free. We read page order here without zone lock
> + * which is generally unsafe, but the race window is small and
> + * the worst thing that can happen is that we skip some
> + * potential isolation targets.

Should we only be doing the low_pfn adjustment based on the order for
MIGRATE_ASYNC? It seems like sync compaction, including compaction that
is triggered from the command line, would prefer to scan over the
following pages.

> */
> - if (PageBuddy(page))
> + if (PageBuddy(page)) {
> + unsigned long freepage_order = page_order_unsafe(page);

I don't assume that we want a smp_wmb() in set_page_order() for this
little race and to recheck PageBuddy() here after smp_rmb().

I think this is fine for MIGRATE_ASYNC.

> +
> + if (freepage_order > 0 && freepage_order < MAX_ORDER)
> + low_pfn += (1UL << freepage_order) - 1;
> continue;
> + }
>
> /*
> * Check may be lockless but that's ok as we recheck later.
> @@ -733,6 +740,13 @@ next_pageblock:
> low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
> }
>
> + /*
> + * The PageBuddy() check could have potentially brought us outside
> + * the range to be scanned.
> + */
> + if (unlikely(low_pfn > end_pfn))
> + end_pfn = low_pfn;
> +
> acct_isolated(zone, locked, cc);
>
> if (locked)
> diff --git a/mm/internal.h b/mm/internal.h
> index 1a8a0d4..6aa1f74 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> * general, page_zone(page)->lock must be held by the caller to prevent the
> * page from being allocated in parallel and returning garbage as the order.
> * If a caller does not hold page_zone(page)->lock, it must guarantee that the
> - * page cannot be allocated or merged in parallel.
> + * page cannot be allocated or merged in parallel. Alternatively, it must
> + * handle invalid values gracefully, and use page_order_unsafe() below.
> */
> static inline unsigned long page_order(struct page *page)
> {
> @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page *page)
> return page_private(page);
> }
>
> +/*
> + * Like page_order(), but for callers who cannot afford to hold the zone lock,
> + * and handle invalid values gracefully. ACCESS_ONCE is used so that if the
> + * caller assigns the result into a local variable and e.g. tests it for valid
> + * range before using, the compiler cannot decide to remove the variable and
> + * inline the function multiple times, potentially observing different values
> + * in the tests and the actual use of the result.
> + */
> +static inline unsigned long page_order_unsafe(struct page *page)
> +{
> + /*
> + * PageBuddy() should be checked by the caller to minimize race window,
> + * and invalid values must be handled gracefully.
> + */
> + return ACCESS_ONCE(page_private(page));
> +}
> +
> /* mm/util.c */
> void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
> struct vm_area_struct *prev, struct rb_node *rb_parent);

I don't like this change at all, I don't think we should have header
functions that imply the context in which the function will be called. I
think it would make much more sense to just do
ACCESS_ONCE(page_order(page)) in the migration scanner with a comment.
These are __attribute__((pure)) semantics for page_order().