[PATCH v3 2/3] fs/file.c: conditionally clear full_fds

[Yu Ma <yu.ma@intel.com>]

64 bits in open_fds are mapped to a common bit in full_fds_bits. It is very
likely that a bit in full_fds_bits has been cleared before in
__clear_open_fds()'s operation. Check the clear bit in full_fds_bits before
clearing to avoid unnecessary write and cache bouncing. See commit fc90888d07b8
("vfs: conditionally clear close-on-exec flag") for a similar optimization.
Take stock kernel with patch 1 as baseline, it improves pts/blogbench-1.1.0
read for 13%, and write for 5% on Intel ICX 160 cores configuration with
v6.10-rc6.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Yu Ma <yu.ma@intel.com>
---
 fs/file.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index 5178b246e54b..a15317db3119 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -268,7 +268,9 @@ static inline void __set_open_fd(unsigned int fd, struct fdtable *fdt)
 static inline void __clear_open_fd(unsigned int fd, struct fdtable *fdt)
 {
 	__clear_bit(fd, fdt->open_fds);
-	__clear_bit(fd / BITS_PER_LONG, fdt->full_fds_bits);
+	fd /= BITS_PER_LONG;
+	if (test_bit(fd, fdt->full_fds_bits))
+		__clear_bit(fd, fdt->full_fds_bits);
 }
 
 static inline bool fd_is_open(unsigned int fd, const struct fdtable *fdt)
-- 
2.43.0