linux-kernel - Re: [PATCH 2/2] fs: make several inode lock operations killable

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Message-ID: <hzrj5b7x3rvtxt4qgjxdihhi5vjoc5gw3i35pbyopa7ccucizo@q5c42kjlkly3>
Date: Mon, 12 May 2025 19:33:13 +0200
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: Max Kellermann <max.kellermann@...os.com>, jack@...e.cz, 
	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs: make several inode lock operations killable

On Mon 12-05-25 11:52:14, Christian Brauner wrote:
> On Tue, Apr 29, 2025 at 01:28:49PM +0200, Max Kellermann wrote:
> > On Tue, Apr 29, 2025 at 1:12 PM Christian Brauner <brauner@...nel.org> wrote:
> > > > --- a/fs/read_write.c
> > > > +++ b/fs/read_write.c
> > > > @@ -332,7 +332,9 @@ loff_t default_llseek(struct file *file, loff_t offset, int whence)
> > > >       struct inode *inode = file_inode(file);
> > > >       loff_t retval;
> > > >
> > > > -     inode_lock(inode);
> > > > +     retval = inode_lock_killable(inode);
> > >
> > > That change doesn't seem so obviously fine to me.
> > 
> > Why do you think so? And how is this different than the other two.
> 
> chown_common() and chmod_common() are very close to the syscall boundary
> so it's very unlikely that we run into weird issues apart from userspace
> regression when they suddenly fail a change for new unexpected reasons.
> 
> But just look at default_llseek():
> 
>     > git grep default_llseek | wc -l
>     461
> 
> That is a lot of stuff and it's not immediately clear how deeply or
> nested they are called. For example from overlayfs in stacked
> callchains. Who knows what strange assumptions some of the callers have
> including the possible return values from that helper.
> 
> > 
> > > Either way I'd like to see this split in three patches and some
> > > reasoning why it's safe and some justification why it's wanted...
> > 
> > Sure I can split this patch, but before I spend the time, I'd like us
> > first to agree that the patch is useful.
> 
> This is difficult to answer. Yes, on the face of it it seems useful to
> be able to kill various operations that sleep on inode lock but who
> knows what implicit guarantees/expectations we're going to break if we
> do it. Maybe @Jan has some thoughts here as well.

So I think having lock killable is useful and generally this should be safe
wrt userspace (the process will get killed before it can notice the
difference anyway). The concern about guarantees / expectations is still
valid for the *kernel* code (which is I think what you meant above). So I
guess some audit of kernel paths that can end up calling ->llseek handler
and whether they are OK with the handler returning EINTR is needed. I
expect this will be fine but I would not bet too much on it :)

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