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

[Jan Kara <jack@suse.cz>]

On Thu 04-07-24 22:45:32, Ma, Yu wrote:
> 
> On 7/4/2024 6:11 PM, Jan Kara wrote:
> > 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@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?
> > 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
> 
> Do you mean that we return 0 here is fd >=end, I'm afraid that might broke
> the original design of this function. And all the callers of it are using
> ret < 0 for error handling, if ret=0, that should mean the fd allocated is 0
> ...

What I meant is that after your changes alloc_fd() could return 0 in fd >=
end case. It could happen if we went through expand_files() which then
returned 0. Anyway, please just fix the ordering of checks and we should be
fine.

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