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

From: Vlastimil Babka
Date: Thu Jun 05 2014 - 05:24:41 EST

On 06/05/2014 02:02 AM, David Rientjes wrote:
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.

I thought even sync compaction would benefit from the skipped iterations. I'd say the probability of this race is smaller than probability of somebody allocating what compaction just freed.

- 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().

Hm right, barriers didn't came up last time a patch like this was posted. Rechecking PageBuddy() did came up but I thought the range checks on the order are enough for this case.

I think this is fine for MIGRATE_ASYNC.

+ if (freepage_order > 0 && freepage_order < MAX_ORDER)
+ low_pfn += (1UL << freepage_order) - 1;
+ }

* 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.

But that won't compile. It would have to be converted to a #define, unless there's some trick I don't know. Sure I would hope this could be done cleaner somehow.

These are __attribute__((pure)) semantics for page_order().

Not sure I understand what you mean here. Would adding that attribute change anything?

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