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

Jan Kara
2024-07-04T10:11:06Z

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