f2fs: don't sleep while grabing nat_tree_lock

This tries to fix priority inversion in the below condition resulting in
long checkpoint delay.

f2fs_get_node_info()
 - nat_tree_lock
  -> sleep to grab journal_rwsem by contention

                                     checkpoint
                                     - waiting for nat_tree_lock

In order to let checkpoint go, let's release nat_tree_lock, if there's a
journal_rwsem contention.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 0be9e2d..c945a97 100644 (file)
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -552,7 +552,7 @@ int f2fs_get_node_info(struct f2fs_sb_info *sbi, nid_t nid,
        int i;
 
        ni->nid = nid;
-
+retry:
        /* Check nat cache */
        down_read(&nm_i->nat_tree_lock);
        e = __lookup_nat_cache(nm_i, nid);
@@ -564,10 +564,19 @@ int f2fs_get_node_info(struct f2fs_sb_info *sbi, nid_t nid,
                return 0;
        }
 
-       memset(&ne, 0, sizeof(struct f2fs_nat_entry));
+       /*
+        * Check current segment summary by trying to grab journal_rwsem first.
+        * This sem is on the critical path on the checkpoint requiring the above
+        * nat_tree_lock. Therefore, we should retry, if we failed to grab here
+        * while not bothering checkpoint.
+        */
+       if (!rwsem_is_locked(&sbi->cp_global_sem)) {
+               down_read(&curseg->journal_rwsem);
+       } else if (!down_read_trylock(&curseg->journal_rwsem)) {
+               up_read(&nm_i->nat_tree_lock);
+               goto retry;
+       }
 
-       /* Check current segment summary */
-       down_read(&curseg->journal_rwsem);
        i = f2fs_lookup_journal_in_cursum(journal, NAT_JOURNAL, nid, 0);
        if (i >= 0) {
                ne = nat_in_journal(journal, i);