[PATCH] f2fs: support to disable linear lookup fallback

[Chao Yu]

After commit 91b587ba79e1 ("f2fs: Introduce linear search for
dentries"), f2fs forced to use linear lookup whenever a hash-based
lookup fails on casefolded directory, it may affect performance
for scenarios: a) create a new file w/ filename it doesn't exist
in directory, b) lookup a file which may be removed.

This patch supports to disable linear lookup fallback, so, once there
is a solution for commit 5c26d2f1d3f5 ("unicode: Don't special case
ignorable code points") to fix red heart unicode issue, then we can
set an encodeing flag to disable the fallback for performance recovery.

The way is kept in line w/ ext4, refer to commit 9e28059d5664 ("ext4:
introduce linear search for dentries").

Cc: Daniel Lee <chullee@xxxxxxxxxx>
Cc: Gabriel Krisman Bertazi <krisman@xxxxxxx>
Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
---
 fs/f2fs/dir.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 5a63ff0df03b..e12445afb95a 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -366,7 +366,8 @@ struct f2fs_dir_entry *__f2fs_find_entry(struct inode *dir,
 
 out:
 #if IS_ENABLED(CONFIG_UNICODE)
-	if (IS_CASEFOLDED(dir) && !de && use_hash) {
+	if (IS_CASEFOLDED(dir) && !de && use_hash &&
+		!sb_no_casefold_compat_fallback(dir->i_sb)) {
 		use_hash = false;
 		goto start_find_entry;
 	}
-- 
2.49.0