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Date:   Thu, 22 Jul 2021 10:44:28 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [PATCH v2] f2fs: don't sleep while grabing nat_tree_lock
Message-ID: <YPmufPvn9FJXfcip@google.com>
References: <20210722014149.525166-1-jaegeuk@kernel.org>
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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: Jaegeuk Kim <jaegeuk@kernel.org>
---
 Change log from v1:
  - drop rwlock but take retry logic to reduce lock-holding time

 fs/f2fs/node.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 0be9e2d7120e..c60ba4179bb2 100644
--- 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 +564,16 @@ 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.
+	 */
+	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);
-- 
2.32.0.432.gabb21c7263-goog