linux-kernel - Re: [PATCH v3 1/3] fs/file.c: remove sanity_check and add likely/unlikely in alloc

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Message-ID: <20240704101104.btkwwnhwf3mnfsvj@quack3>
Date: Thu, 4 Jul 2024 12:11:04 +0200
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: Yu Ma <yu.ma@...el.com>, viro@...iv.linux.org.uk, jack@...e.cz,
	mjguzik@...il.com, edumazet@...gle.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	pan.deng@...el.com, tianyou.li@...el.com, tim.c.chen@...el.com,
	tim.c.chen@...ux.intel.com
Subject: Re: [PATCH v3 1/3] fs/file.c: remove sanity_check and add
 likely/unlikely in alloc_fd()

On Wed 03-07-24 16:34:49, 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@...ux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@...el.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?

Yeah, not only that but also:

> >  	/*
> >  	 * 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;

We could then exit here with error set to 0 instead of -EMFILE. So this
needs a bit of work. But otherwise the patch looks good to me.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR