mm/compaction: remove rcu_read_lock during page compaction

isolate_migratepages_block() used rcu_read_lock() with the intention of
safeguarding against the mem_cgroup being destroyed concurrently; but
its TestClearPageLRU already protects against that.  Delete the
unnecessary rcu_read_lock() and _unlock().

Hugh Dickins helped on commit log polishing, Thanks!

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alex Shi <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/compaction.c

@@ -995,7 +995,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (!TestClearPageLRU(page))
 			goto isolate_fail_put;
 
-		rcu_read_lock();
 		lruvec = mem_cgroup_page_lruvec(page, pgdat);
 
 		/* If we already hold the lock, we can skip some rechecking */
@@ -1005,7 +1004,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 
 			compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
 			locked = lruvec;
-			rcu_read_unlock();
 
 			lruvec_memcg_debug(lruvec, page);
 
@@ -1026,8 +1024,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				SetPageLRU(page);
 				goto isolate_fail_put;
 			}
-		} else
-			rcu_read_unlock();
+		}
 
 		/* The whole page is taken off the LRU; skip the tail pages. */
 		if (PageCompound(page))