linux-kernel - Re: [PATCH 3/3] fs/file.c: move sanity_check from alloc_fd() to put_unused

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Date: Mon, 17 Jun 2024 10:36:03 +0200
From: Christian Brauner <brauner@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Yu Ma <yu.ma@...el.com>, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com, 
	tim.c.chen@...el.com, pan.deng@...el.com, tianyou.li@...el.com
Subject: Re: [PATCH 3/3] fs/file.c: move sanity_check from alloc_fd() to
 put_unused_fd()

On Sat, Jun 15, 2024 at 06:41:45AM GMT, Mateusz Guzik wrote:
> On Fri, Jun 14, 2024 at 12:34:16PM -0400, Yu Ma wrote:
> > alloc_fd() has a sanity check inside to make sure the FILE object mapping to the
> 
> Total nitpick: FILE is the libc thing, I would refer to it as 'struct
> file'. See below for the actual point.
> 
> > Combined with patch 1 and 2 in series, pts/blogbench-1.1.0 read improved by
> > 32%, write improved by 15% on Intel ICX 160 cores configuration with v6.8-rc6.
> > 
> > Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@...el.com>
> > ---
> >  fs/file.c | 14 ++++++--------
> >  1 file changed, 6 insertions(+), 8 deletions(-)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index a0e94a178c0b..59d62909e2e3 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -548,13 +548,6 @@ 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) {
> > -		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
> > -		rcu_assign_pointer(fdt->fd[fd], NULL);
> > -	}
> > -#endif
> >  
> 
> I was going to ask when was the last time anyone seen this fire and
> suggest getting rid of it if enough time(tm) passed. Turns out it does
> show up sometimes, latest result I found is 2017 vintage:
> https://groups.google.com/g/syzkaller-bugs/c/jfQ7upCDf9s/m/RQjhDrZ7AQAJ
> 
> So you are moving this to another locked area, but one which does not
> execute in the benchmark?
> 
> Patch 2/3 states 28% read and 14% write increase, this commit message
> claims it goes up to 32% and 15% respectively -- pretty big. I presume
> this has to do with bouncing a line containing the fd.
> 
> I would argue moving this check elsewhere is about as good as removing
> it altogether, but that's for the vfs overlords to decide.

This all dates back to 9d11a5176cc5 ("I just put a pre-90 on
ftp.kernel.org, and I'm happy to report that Davem")
which is pre-git. I think removing is fine.