Re: [PATCH v3 1/3] fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()

Christian Brauner
2024-07-03T14:34:56Z

On Wed, Jul 03, 2024 at 10:33:09AM GMT, Yu Ma wrote:
> alloc_fd() has a sanity check inside to make sure the struct file mapping to the
> allocated fd is NULL. Remove this sanity check since it can be assured by
> exisitng zero initilization and NULL set when recycling fd. Meanwhile, add
> likely/unlikely and expand_file() call avoidance to reduce the work under
> file_lock.
>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@intel.com>
> ---
> fs/file.c | 38 ++++++++++++++++----------------------
> 1 file changed, 16 insertions(+), 22 deletions(-)
>
> diff --git a/fs/file.c b/fs/file.c
> index a3b72aa64f11..5178b246e54b 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -515,28 +515,29 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
> if (fd < files->next_fd)
> fd = files->next_fd;
>
> - if (fd < fdt->max_fds)
> + if (likely(fd < fdt->max_fds))
> fd = find_next_fd(fdt, fd);
>
> + error = -EMFILE;
> + if (unlikely(fd >= fdt->max_fds)) {
> + error = expand_files(files, fd);
> + if (error < 0)
> + goto out;
> + /*
> + * If we needed to expand the fs array we
> + * might have blocked - try again.
> + */
> + if (error)
> + goto repeat;
> + }

So this ends up removing the expand_files() above the fd >= end check
which means that you can end up expanding the files_struct even though
the request fd is past the provided end. That seems odd. What's the
reason for that reordering?

> +
> /*
> * N.B. For clone tasks sharing a files structure, this test
> * will limit the total number of files that can be opened.
> */
> - error = -EMFILE;
> - if (fd >= end)
> - goto out;
> -
> - error = expand_files(files, fd);
> - if (error < 0)
> + if (unlikely(fd >= end))
> goto out;
>
> - /*
> - * If we needed to expand the fs array we
> - * might have blocked - try again.
> - */
> - if (error)
> - goto repeat;
> -
> if (start <= files->next_fd)
> files->next_fd = fd + 1;
>
> @@ -546,13 +547,6 @@ 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);
> @@ -618,7 +612,7 @@ void fd_install(unsigned int fd, struct file *file)
> rcu_read_unlock_sched();
> spin_lock(&files->file_lock);
> fdt = files_fdtable(files);
> - BUG_ON(fdt->fd[fd] != NULL);
> + WARN_ON(fdt->fd[fd] != NULL);
> rcu_assign_pointer(fdt->fd[fd], file);
> spin_unlock(&files->file_lock);
> return;
> --
> 2.43.0
>