[PATCH 3/3] fs/file.c: move sanity_check from alloc_fd() to put_unused_fd()

[Yu Ma]

alloc_fd() has a sanity check inside to make sure the FILE object mapping to the
allocated fd is NULL. Move the sanity check from performance critical alloc_fd()
path to non performance critical put_unused_fd() path.

As the initial NULL FILE object condition can be assured by zero initialization
in init_file, we just need to make sure that it is NULL when recycling fd back.
There are 3 functions call __put_unused_fd() to return fd,
file_close_fd_locked(), do_close_on_exec() and put_unused_fd(). For
file_close_fd_locked() and do_close_on_exec(), they have implemented NULL check
already. Adds NULL check to put_unused_fd() to cover all release paths.

Combined with patch 1 and 2 in series, pts/blogbench-1.1.0 read improved by
32%, write improved by 15% on Intel ICX 160 cores configuration with v6.8-rc6.

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
---
 fs/file.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index a0e94a178c0b..59d62909e2e3 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -548,13 +548,6 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 	else
 		__clear_close_on_exec(fd, fdt);
 	error = fd;
-#if 1
-	/* Sanity check */
-	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
-		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
-		rcu_assign_pointer(fdt->fd[fd], NULL);
-	}
-#endif
 
 out:
 	spin_unlock(&files->file_lock);
@@ -572,7 +565,7 @@ int get_unused_fd_flags(unsigned flags)
 }
 EXPORT_SYMBOL(get_unused_fd_flags);
 
-static void __put_unused_fd(struct files_struct *files, unsigned int fd)
+static inline void __put_unused_fd(struct files_struct *files, unsigned int fd)
 {
 	struct fdtable *fdt = files_fdtable(files);
 	__clear_open_fd(fd, fdt);
@@ -583,7 +576,12 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
 void put_unused_fd(unsigned int fd)
 {
 	struct files_struct *files = current->files;
+	struct fdtable *fdt = files_fdtable(files);
 	spin_lock(&files->file_lock);
+	if (unlikely(rcu_access_pointer(fdt->fd[fd]))) {
+		printk(KERN_WARNING "put_unused_fd: slot %d not NULL!\n", fd);
+		rcu_assign_pointer(fdt->fd[fd], NULL);
+	}
 	__put_unused_fd(files, fd);
 	spin_unlock(&files->file_lock);
 }
-- 
2.43.0