Re: [PATCH v2 3/3] fs/file.c: remove sanity_check from alloc_fd()

[Christian Brauner <brauner@kernel.org>]

On Tue, Jun 25, 2024 at 03:30:31PM GMT, Jan Kara wrote:
> On Tue 25-06-24 15:11:23, Mateusz Guzik wrote:
> > On Tue, Jun 25, 2024 at 3:09 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
> > >
> > > On Tue, Jun 25, 2024 at 2:08 PM Jan Kara <jack@suse.cz> wrote:
> > > >
> > > > On Sat 22-06-24 11:49:04, 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.
> > > >   ^^^ existing  ^^^ initialization
> > > >
> > > > Well, since this is a sanity check, it is expected it never hits. Yet
> > > > searching the web shows it has hit a few times in the past :). So would
> > > > wrapping this with unlikely() give a similar performance gain while keeping
> > > > debugability? If unlikely() does not help, I agree we can remove this since
> > > > fd_install() actually has the same check:
> > > >
> > > > BUG_ON(fdt->fd[fd] != NULL);
> > > >
> > > > and there we need the cacheline anyway so performance impact is minimal.
> > > > Now, this condition in alloc_fd() is nice that it does not take the kernel
> > > > down so perhaps we could change the BUG_ON to WARN() dumping similar kind
> > > > of info as alloc_fd()?
> > > >
> > >
> > > Christian suggested just removing it.
> > >
> > > To my understanding the problem is not the branch per se, but the the
> > > cacheline bounce of the fd array induced by reading the status.
> > >
> > > Note the thing also nullifies the pointer, kind of defeating the
> > > BUG_ON in fd_install.
> > >
> > > I'm guessing it's not going to hurt to branch on it after releasing
> > > the lock and forego nullifying, more or less:
> > > diff --git a/fs/file.c b/fs/file.c
> > > index a3b72aa64f11..d22b867db246 100644
> > > --- a/fs/file.c
> > > +++ b/fs/file.c
> > > @@ -524,11 +524,11 @@ static int alloc_fd(unsigned start, unsigned
> > > end, unsigned flags)
> > >          */
> > >         error = -EMFILE;
> > >         if (fd >= end)
> > > -               goto out;
> > > +               goto out_locked;
> > >
> > >         error = expand_files(files, fd);
> > >         if (error < 0)
> > > -               goto out;
> > > +               goto out_locked;
> > >
> > >         /*
> > >          * If we needed to expand the fs array we
> > > @@ -546,15 +546,15 @@ 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) {
> > > +       spin_unlock(&files->file_lock);
> > > +
> > > +       if (unlikely(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
> > 
> > Now that I sent it it is of course not safe to deref without
> > protection from either rcu or the lock, so this would have to be
> > wrapped with rcu_read_lock, which makes it even less appealing.
> > 
> > Whacking the thing as in the submitted patch seems like the best way
> > forward here. :)
> 
> Yeah, as I wrote, I'm fine removing it, in particular if Christian is of
> the same opinion. I was more musing about whether we should make the check
> in fd_install() less aggressive since it is now more likely to trigger...

We could change it to WARN_ON() and then people can get BUG_ON()
behavior when they turn WARN into BUG which apparently is a thing that
we support.