From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Ian Kent <raven@themaw.net>
Cc: Miklos Szeredi <miklos@szeredi.hu>, Tejun Heo <tj@kernel.org>,
Eric Sandeen <sandeen@sandeen.net>,
Fox Chen <foxhlchen@gmail.com>,
Brice Goglin <brice.goglin@gmail.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Rick Lindsley <ricklind@linux.vnet.ibm.com>,
David Howells <dhowells@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Carlos Maiolino <cmaiolino@redhat.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 2/7] kernfs: add a revision to identify directory node changes
Date: Fri, 11 Jun 2021 16:05:11 +0200 [thread overview]
Message-ID: <YMNtl9sSwQ9bPENA@kroah.com> (raw)
In-Reply-To: <21ec3ad11c4d0d74f9b51df3c3e43ab9f62c32b4.camel@themaw.net>
On Fri, Jun 11, 2021 at 09:31:36PM +0800, Ian Kent wrote:
> On Fri, 2021-06-11 at 15:11 +0200, Greg Kroah-Hartman wrote:
> > On Fri, Jun 11, 2021 at 08:56:18PM +0800, Ian Kent wrote:
> > > On Fri, 2021-06-11 at 14:49 +0200, Miklos Szeredi wrote:
> > > > On Wed, 9 Jun 2021 at 10:50, Ian Kent <raven@themaw.net> wrote:
> > > > >
> > > > > Add a revision counter to kernfs directory nodes so it can be
> > > > > used
> > > > > to detect if a directory node has changed during negative
> > > > > dentry
> > > > > revalidation.
> > > > >
> > > > > There's an assumption that sizeof(unsigned long) <=
> > > > > sizeof(pointer)
> > > > > on all architectures and as far as I know that assumption
> > > > > holds.
> > > > >
> > > > > So adding a revision counter to the struct kernfs_elem_dir
> > > > > variant
> > > > > of
> > > > > the kernfs_node type union won't increase the size of the
> > > > > kernfs_node
> > > > > struct. This is because struct kernfs_elem_dir is at least
> > > > > sizeof(pointer) smaller than the largest union variant. It's
> > > > > tempting
> > > > > to make the revision counter a u64 but that would increase the
> > > > > size
> > > > > of
> > > > > kernfs_node on archs where sizeof(pointer) is smaller than the
> > > > > revision
> > > > > counter.
> > > > >
> > > > > Signed-off-by: Ian Kent <raven@themaw.net>
> > > > > ---
> > > > > fs/kernfs/dir.c | 2 ++
> > > > > fs/kernfs/kernfs-internal.h | 23 +++++++++++++++++++++++
> > > > > include/linux/kernfs.h | 5 +++++
> > > > > 3 files changed, 30 insertions(+)
> > > > >
> > > > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> > > > > index 33166ec90a112..b3d1bc0f317d0 100644
> > > > > --- a/fs/kernfs/dir.c
> > > > > +++ b/fs/kernfs/dir.c
> > > > > @@ -372,6 +372,7 @@ static int kernfs_link_sibling(struct
> > > > > kernfs_node *kn)
> > > > > /* successfully added, account subdir number */
> > > > > if (kernfs_type(kn) == KERNFS_DIR)
> > > > > kn->parent->dir.subdirs++;
> > > > > + kernfs_inc_rev(kn->parent);
> > > > >
> > > > > return 0;
> > > > > }
> > > > > @@ -394,6 +395,7 @@ static bool kernfs_unlink_sibling(struct
> > > > > kernfs_node *kn)
> > > > >
> > > > > if (kernfs_type(kn) == KERNFS_DIR)
> > > > > kn->parent->dir.subdirs--;
> > > > > + kernfs_inc_rev(kn->parent);
> > > > >
> > > > > rb_erase(&kn->rb, &kn->parent->dir.children);
> > > > > RB_CLEAR_NODE(&kn->rb);
> > > > > diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-
> > > > > internal.h
> > > > > index ccc3b44f6306f..b4e7579e04799 100644
> > > > > --- a/fs/kernfs/kernfs-internal.h
> > > > > +++ b/fs/kernfs/kernfs-internal.h
> > > > > @@ -81,6 +81,29 @@ static inline struct kernfs_node
> > > > > *kernfs_dentry_node(struct dentry *dentry)
> > > > > return d_inode(dentry)->i_private;
> > > > > }
> > > > >
> > > > > +static inline void kernfs_set_rev(struct kernfs_node *kn,
> > > > > + struct dentry *dentry)
> > > > > +{
> > > > > + if (kernfs_type(kn) == KERNFS_DIR)
> > > > > + dentry->d_time = kn->dir.rev;
> > > > > +}
> > > > > +
> > > > > +static inline void kernfs_inc_rev(struct kernfs_node *kn)
> > > > > +{
> > > > > + if (kernfs_type(kn) == KERNFS_DIR)
> > > > > + kn->dir.rev++;
> > > > > +}
> > > > > +
> > > > > +static inline bool kernfs_dir_changed(struct kernfs_node *kn,
> > > > > + struct dentry *dentry)
> > > > > +{
> > > > > + if (kernfs_type(kn) == KERNFS_DIR) {
> > > >
> > > > Aren't these always be called on a KERNFS_DIR node?
> > >
> > > Yes they are.
> > >
> > > >
> > > > You could just reduce that to a WARN_ON, or remove the conditions
> > > > altogether then.
> > >
> > > I was tempted to not use the check, a WARN_ON sounds better than
> > > removing the check, I'll do that in a v7.
> >
> > No, WARN_ON is not ok, as systems will crash if panic-on-warn is set.
>
> Thanks Greg, understood.
>
> >
> > If these are impossible to hit, great, let's not check this and we
> > can
> > just drop the code. If they can be hit, then the above code is
> > correct
> > and it should stay.
>
> It's a programming mistake to call these on a non-directory node.
>
> I can remove the check but do you think there's any value in passing
> the node and updating it's parent to avoid possible misuse?
I do not understand the question here, sorry. It's a static function,
you control the callers, who can "misuse" it?
thanks,
greg k-h
next prev parent reply other threads:[~2021-06-11 14:05 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 8:49 [PATCH v6 0/7] kernfs: proposed locking and concurrency improvement Ian Kent
2021-06-09 8:49 ` [PATCH v6 1/7] kernfs: move revalidate to be near lookup Ian Kent
2021-06-11 12:45 ` Miklos Szeredi
2021-06-09 8:49 ` [PATCH v6 2/7] kernfs: add a revision to identify directory node changes Ian Kent
2021-06-11 12:49 ` Miklos Szeredi
2021-06-11 12:56 ` Ian Kent
2021-06-11 13:11 ` Greg Kroah-Hartman
2021-06-11 13:31 ` Ian Kent
2021-06-11 14:05 ` Greg Kroah-Hartman [this message]
2021-06-11 14:16 ` Ian Kent
2021-06-09 8:50 ` [PATCH v6 3/7] kernfs: use VFS negative dentry caching Ian Kent
2021-06-11 13:07 ` Miklos Szeredi
2021-06-12 0:47 ` Ian Kent
2021-06-12 1:48 ` Al Viro
2021-06-13 1:16 ` Ian Kent
2021-06-12 0:07 ` Al Viro
2021-06-12 0:43 ` Ian Kent
2021-06-12 1:08 ` Ian Kent
2021-06-12 1:51 ` Al Viro
2021-06-13 1:57 ` Ian Kent
2021-06-09 8:50 ` [PATCH v6 4/7] kernfs: switch kernfs to use an rwsem Ian Kent
2021-06-11 13:10 ` Miklos Szeredi
2021-06-12 1:24 ` Al Viro
2021-06-09 8:51 ` [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates Ian Kent
2021-06-11 13:11 ` Miklos Szeredi
2021-06-12 1:45 ` Al Viro
2021-06-13 1:31 ` Ian Kent
2021-06-14 1:32 ` Ian Kent
2021-06-14 6:52 ` Ian Kent
2021-06-14 7:16 ` Ian Kent
2021-06-09 8:52 ` [PATCH v6 6/7] kernfs: add kernfs_need_inode_refresh() Ian Kent
2021-06-11 13:13 ` Miklos Szeredi
2021-06-09 8:52 ` [PATCH v6 7/7] kernfs: dont call d_splice_alias() under kernfs node lock Ian Kent
2021-06-11 13:14 ` Miklos Szeredi
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