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/1608614453-10739-3-git-send-email-alex.shi@linux.alibaba.com
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/compaction.c b/mm/compaction.c
index 3aef10c61d0eca330b383f336ecc3570ed0b2a1f..69ca3d6f97644d9343cc95b115f5e56e499f3f55 100644 (file)
--- a/mm/compaction.c
+++ b/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))