Re: [PATCH] fs/lock: Don't allocate file_lock in flock_make_lock().

[Chuck Lever III <chuck.lever@oracle.com>]

> On Jul 15, 2022, at 9:31 PM, Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> 
> Two functions, flock syscall and locks_remove_flock(), call
> flock_make_lock().  It allocates struct file_lock from slab
> cache if its argument fl is NULL.
> 
> When we call flock syscall, we pass NULL to allocate memory
> for struct file_lock.  However, we always free it at the end
> by locks_free_lock().  We need not allocate it and instead
> should use a local variable as locks_remove_flock() does.
> 
> Also, the validation for flock_translate_cmd() is not necessary
> for locks_remove_flock().  So we move the part to flock syscall
> and make flock_make_lock() return nothing.
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

It looks like a reasonable clean-up. Handful of comments below.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>


> ---
> fs/locks.c | 57 +++++++++++++++++++-----------------------------------
> 1 file changed, 20 insertions(+), 37 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index ca28e0e50e56..db75f4537abc 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -425,21 +425,9 @@ static inline int flock_translate_cmd(int cmd) {
> }
> 
> /* Fill in a file_lock structure with an appropriate FLOCK lock. */
> -static struct file_lock *
> -flock_make_lock(struct file *filp, unsigned int cmd, struct file_lock *fl)
> +static void flock_make_lock(struct file *filp, struct file_lock *fl, int type)
> {
> -	int type = flock_translate_cmd(cmd);
> -
> -	if (type < 0)
> -		return ERR_PTR(type);
> -
> -	if (fl == NULL) {
> -		fl = locks_alloc_lock();
> -		if (fl == NULL)
> -			return ERR_PTR(-ENOMEM);
> -	} else {
> -		locks_init_lock(fl);
> -	}
> +	locks_init_lock(fl);
> 
> 	fl->fl_file = filp;
> 	fl->fl_owner = filp;
> @@ -447,8 +435,6 @@ flock_make_lock(struct file *filp, unsigned int cmd, struct file_lock *fl)
> 	fl->fl_flags = FL_FLOCK;
> 	fl->fl_type = type;
> 	fl->fl_end = OFFSET_MAX;
> -
> -	return fl;
> }
> 
> static int assign_type(struct file_lock *fl, long type)
> @@ -2097,14 +2083,18 @@ EXPORT_SYMBOL(locks_lock_inode_wait);
>  */
> SYSCALL_DEFINE2(flock, unsigned int, fd, unsigned int, cmd)
> {
> -	struct fd f = fdget(fd);
> -	struct file_lock *lock;
> -	int can_sleep, unlock;
> +	int can_sleep, unlock, type;
> +	struct file_lock fl;
> +	struct fd f;
> 	int error;

"struct file_lock" on my system is 216 bytes. That's a lot to
allocate on the stack, but there isn't much else there in
addition to "struct file_lock", so OK.


> -	error = -EBADF;
> +	type = flock_translate_cmd(cmd);
> +	if (type < 0)
> +		return type;
> +
> +	f = fdget(fd);
> 	if (!f.file)
> -		goto out;
> +		return -EBADF;
> 
> 	can_sleep = !(cmd & LOCK_NB);
> 	cmd &= ~LOCK_NB;
> @@ -2127,32 +2117,25 @@ SYSCALL_DEFINE2(flock, unsigned int, fd, unsigned int, cmd)
> 		goto out_putf;
> 	}
> 
> -	lock = flock_make_lock(f.file, cmd, NULL);
> -	if (IS_ERR(lock)) {
> -		error = PTR_ERR(lock);
> -		goto out_putf;
> -	}
> +	flock_make_lock(f.file, &fl, type);
> 
> 	if (can_sleep)
> -		lock->fl_flags |= FL_SLEEP;
> +		fl.fl_flags |= FL_SLEEP;
> 
> -	error = security_file_lock(f.file, lock->fl_type);
> +	error = security_file_lock(f.file, fl.fl_type);
> 	if (error)
> -		goto out_free;
> +		goto out_putf;
> 
> 	if (f.file->f_op->flock)
> 		error = f.file->f_op->flock(f.file,
> -					  (can_sleep) ? F_SETLKW : F_SETLK,
> -					  lock);
> +					    can_sleep ? F_SETLKW : F_SETLK,
> +					    &fl);
> 	else
> -		error = locks_lock_file_wait(f.file, lock);
> -
> - out_free:
> -	locks_free_lock(lock);
> +		error = locks_lock_file_wait(f.file, &fl);
> 
>  out_putf:
> 	fdput(f);
> - out:
> +
> 	return error;
> }
> 
> @@ -2614,7 +2597,7 @@ locks_remove_flock(struct file *filp, struct file_lock_context *flctx)
> 	if (list_empty(&flctx->flc_flock))
> 		return;
> 
> -	flock_make_lock(filp, LOCK_UN, &fl);
> +	flock_make_lock(filp, &fl, flock_translate_cmd(LOCK_UN));

We hope the compiler recognizes that passing a constant value through
a switch statement means the flock_translate_cmd() call here is
reduced to a constant F_UNLCK. It might be slightly easier to read
if you explicitly pass F_UNLCK here? Dunno.


> 	fl.fl_flags |= FL_CLOSE;
> 
> 	if (filp->f_op->flock)
> -- 
> 2.30.2
> 

--
Chuck Lever