linux-kernel - Re: [PATCH] fs: assert on ->i_count in iput

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Message-ID: <CAGudoHFAar2rHaCDWP4uD2QD_BO42-Fi6b9uxwFvHTmkXTCQfA@mail.gmail.com>
Date: Wed, 1 Oct 2025 16:28:15 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: assert on ->i_count in iput_final()

On Wed, Oct 1, 2025 at 3:08 PM Jan Kara <jack@...e.cz> wrote:
>
> On Wed 01-10-25 14:12:13, Mateusz Guzik wrote:
> > On Wed, Oct 1, 2025 at 2:07 PM Jan Kara <jack@...e.cz> wrote:
> > > > diff --git a/fs/inode.c b/fs/inode.c
> > > > index ec9339024ac3..fa82cb810af4 100644
> > > > --- a/fs/inode.c
> > > > +++ b/fs/inode.c
> > > > @@ -1879,6 +1879,7 @@ static void iput_final(struct inode *inode)
> > > >       int drop;
> > > >
> > > >       WARN_ON(inode->i_state & I_NEW);
> > > > +     VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
> > >
> > > This seems pointless given when iput_final() is called...
> > >
> >
> > This and the other check explicitly "wrap" the ->drop_inode call.
>
> I understand but given iput() has just decremented i_count to 0 before
> calling iput_final() this beginning of the "wrap" looks pretty pointless to
> me.
>

To my understanding you are not NAKing the patch, are merely not
particularly fond of it. ;)

Given that these asserts don't show up in production kernels, the
layer should be moving towards always spelling out all assumptions at
the entry point. Worst case does not hurt in production anyway, best
case it will catch something.

For iput_final specifically, at the moment there is only one consumer
so this indeed may look overzealous.

But for the sake of argument suppose someone noticed that
dentry_unlink_inode() performs:
        spin_unlock(&inode->i_lock);
        if (!inode->i_nlink)
                fsnotify_inoderemove(inode);
        if (dentry->d_op && dentry->d_op->d_iput)
                dentry->d_op->d_iput(dentry, inode);
        else
                iput(inode);

... and that with some minor rototoiling the inode lock can survive
both fsnotify and custom d_iput in the common case. Should that
happen, iput_locked() could be added to shave off a lock trip in the
common case of whacking the inode. But then there is 2 consumers of
iput_final. etc.