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

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Date:   Tue, 21 May 2019 18:41:42 +0200
From:   Christian Brauner <christian@...uner.io>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        jannh@...gle.com, fweimer@...hat.com, oleg@...hat.com,
        tglx@...utronix.de, torvalds@...ux-foundation.org, arnd@...db.de,
        shuah@...nel.org, 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 1/2] open: add close_range()

On Tue, May 21, 2019 at 05:30:27PM +0100, David Howells wrote:
> Al Viro <viro@...iv.linux.org.uk> wrote:
> 
> > Umm...  That's going to be very painful if you dup2() something to MAX_INT and
> > then run that; roughly 2G iterations of bouncing ->file_lock up and down,
> > without anything that would yield CPU in process.
> > 
> > If anything, I would suggest something like
> > 
> > 	fd = *start_fd;
> > 	grab the lock
> >         fdt = files_fdtable(files);
> > more:
> > 	look for the next eviction candidate in ->open_fds, starting at fd
> > 	if there's none up to max_fd
> > 		drop the lock
> > 		return NULL
> > 	*start_fd = fd + 1;
> > 	if the fscker is really opened and not just reserved
> > 		rcu_assign_pointer(fdt->fd[fd], NULL);
> > 		__put_unused_fd(files, fd);
> > 		drop the lock
> > 		return the file we'd got
> > 	if (unlikely(need_resched()))
> > 		drop lock
> > 		cond_resched();
> > 		grab lock
> > 		fdt = files_fdtable(files);
> > 	goto more;
> > 
> > with the main loop being basically
> > 	while ((file = pick_next(files, &start_fd, max_fd)) != NULL)
> > 		filp_close(file, files);
> 
> If we can live with close_from(int first) rather than close_range(), then this
> can perhaps be done a lot more efficiently by:

Yeah, you mentioned this before. I do like being able to specify an
upper bound to have the ability to place fds strategically after said
upper bound.
I have used this quite a few times where I know that given task may have
inherited up to m fds and I want to inherit a specific pipe who's fd I
know. Then I'd dup2(pipe_fd, <upper_bound + 1>) and then close all
other fds. Is that too much of a corner case?

Christian