linux-kernel - Re: [PATCH v1 1/2] open: add close

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Date:   Thu, 23 May 2019 16:28:28 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        torvalds@...ux-foundation.org, fweimer@...hat.com,
        jannh@...gle.com, tglx@...utronix.de, arnd@...db.de,
        shuah@...nel.org, dhowells@...hat.com, tkjos@...roid.com,
        ldv@...linux.org, miklos@...redi.hu, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v1 1/2] open: add close_range()

On Thu, May 23, 2019 at 04:14:47PM +0200, Christian Brauner wrote:
> On Thu, May 23, 2019 at 01:51:18PM +0200, Christian Brauner wrote:
> > On Wed, May 22, 2019 at 06:57:37PM +0200, Oleg Nesterov wrote:
> > > On 05/22, Christian Brauner wrote:
> > > >
> > > > +static struct file *pick_file(struct files_struct *files, unsigned fd)
> > > >  {
> > > > -	struct file *file;
> > > > +	struct file *file = NULL;
> > > >  	struct fdtable *fdt;
> > > >  
> > > >  	spin_lock(&files->file_lock);
> > > > @@ -632,15 +629,65 @@ int __close_fd(struct files_struct *files, unsigned fd)
> > > >  		goto out_unlock;
> > > >  	rcu_assign_pointer(fdt->fd[fd], NULL);
> > > >  	__put_unused_fd(files, fd);
> > > > -	spin_unlock(&files->file_lock);
> > > > -	return filp_close(file, files);
> > > >  
> > > >  out_unlock:
> > > >  	spin_unlock(&files->file_lock);
> > > > -	return -EBADF;
> > > > +	return file;
> > > 
> > > ...
> > > 
> > > > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd)
> > > > +{
> > > > +	unsigned int cur_max;
> > > > +
> > > > +	if (fd > max_fd)
> > > > +		return -EINVAL;
> > > > +
> > > > +	rcu_read_lock();
> > > > +	cur_max = files_fdtable(files)->max_fds;
> > > > +	rcu_read_unlock();
> > > > +
> > > > +	/* cap to last valid index into fdtable */
> > > > +	if (max_fd >= cur_max)
> > > > +		max_fd = cur_max - 1;
> > > > +
> > > > +	while (fd <= max_fd) {
> > > > +		struct file *file;
> > > > +
> > > > +		file = pick_file(files, fd++);
> > > 
> > > Well, how about something like
> > > 
> > > 	static unsigned int find_next_opened_fd(struct fdtable *fdt, unsigned start)
> > > 	{
> > > 		unsigned int maxfd = fdt->max_fds;
> > > 		unsigned int maxbit = maxfd / BITS_PER_LONG;
> > > 		unsigned int bitbit = start / BITS_PER_LONG;
> > > 
> > > 		bitbit = find_next_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
> > > 		if (bitbit > maxfd)
> > > 			return maxfd;
> > > 		if (bitbit > start)
> > > 			start = bitbit;
> > > 		return find_next_bit(fdt->open_fds, maxfd, start);
> > > 	}
> > 
> > > 
> > > 	unsigned close_next_fd(struct files_struct *files, unsigned start, unsigned maxfd)
> > > 	{
> > > 		unsigned fd;
> > > 		struct file *file;
> > > 		struct fdtable *fdt;
> > > 	
> > > 		spin_lock(&files->file_lock);
> > > 		fdt = files_fdtable(files);
> > > 		fd = find_next_opened_fd(fdt, start);
> > > 		if (fd >= fdt->max_fds || fd > maxfd) {
> > > 			fd = -1;
> > > 			goto out;
> > > 		}
> > > 
> > > 		file = fdt->fd[fd];
> > > 		rcu_assign_pointer(fdt->fd[fd], NULL);
> > > 		__put_unused_fd(files, fd);
> > > 	out:
> > > 		spin_unlock(&files->file_lock);
> > > 
> > > 		if (fd == -1u)
> > > 			return fd;
> > > 
> > > 		filp_close(file, files);
> > > 		return fd + 1;
> > > 	}
> > 
> > Thanks, Oleg!
> > 
> > I kept it dumb and was about to reply that your solution introduces more
> > code when it seemed we wanted to keep this very simple for now.
> > But then I saw that find_next_opened_fd() already exists as
> > find_next_fd(). So it's actually not bad compared to what I sent in v1.
> > So - with some small tweaks (need to test it and all now) - how do we
> > feel about?:
> 
> That's obviously not correct atm but I'll send out a tweaked version in
> a bit.

So given that we would really need another find_next_open_fd() I think
sticking to the simple cond_resched() version I sent before is better
for now until we see real-world performance issues.
I was however missing a test for close_range(fd, fd, 0) anyway so I'll
need to send a v2 with this test added.

Christian