[RFC PATCH v4 02/12] mm: memcontrol: introduce compact_lock_page_irqsave

[Muchun Song]

If we reuse the objcg APIs to charge LRU pages, the page_memcg()
can be changed when the LRU pages reparented. In this case, we need
to acquire the new lruvec lock.

    lruvec = mem_cgroup_page_lruvec(page);

    // The page is reparented.

    compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);

    // Acquired the wrong lruvec lock and need to retry.

But compact_lock_irqsave() only take lruvec lock as the parameter,
we cannot aware this change. If it can take the page as parameter
to acquire the lruvec lock. When the page memcg is changed, we can
use the page_memcg() detect whether we need to reacquire the new
lruvec lock. So compact_lock_irqsave() is not suitable for us.
Similar to lock_page_lruvec_irqsave(), introduce
compact_lock_page_irqsave() to acquire the lruvec lock in
the compaction routine.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
---
 mm/compaction.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 7d41b58fb17c..851fd8f62695 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -511,6 +511,29 @@ static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
 	return true;
 }
 
+static struct lruvec *compact_lock_page_irqsave(struct page *page,
+						unsigned long *flags,
+						struct compact_control *cc)
+{
+	struct lruvec *lruvec;
+
+	lruvec = mem_cgroup_page_lruvec(page);
+
+	/* Track if the lock is contended in async mode */
+	if (cc->mode == MIGRATE_ASYNC && !cc->contended) {
+		if (spin_trylock_irqsave(&lruvec->lru_lock, *flags))
+			goto out;
+
+		cc->contended = true;
+	}
+
+	spin_lock_irqsave(&lruvec->lru_lock, *flags);
+out:
+	lruvec_memcg_debug(lruvec, page);
+
+	return lruvec;
+}
+
 /*
  * Compaction requires the taking of some coarse locks that are potentially
  * very heavily contended. The lock should be periodically unlocked to avoid
@@ -1035,11 +1058,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 			if (locked)
 				unlock_page_lruvec_irqrestore(locked, flags);
 
-			compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
+			lruvec = compact_lock_page_irqsave(page, &flags, cc);
 			locked = lruvec;
 
-			lruvec_memcg_debug(lruvec, page);
-
 			/* Try get exclusive access under lock */
 			if (!skip_updated) {
 				skip_updated = true;
-- 
2.11.0