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

[Ma, Yu]

On 7/3/2024 10:34 PM, Christian Brauner wrote:
> 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> > ---
> >   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?

Yes, you are right, thanks Christian. This incorrect reordering here is 
due to historical versions with fast path inside. I'll update the order 
back.

> > +
> >   	/*
> >   	 * 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