Re: [PATCH 04/14] kernfs: adapt to rhashtable-based simple_xattrs with lazy allocation

[Christian Brauner <brauner@kernel.org>]

On Fri, Feb 27, 2026 at 04:00:37PM +0100, Jan Kara wrote:
> On Mon 16-02-26 14:32:00, Christian Brauner wrote:
> > Adapt kernfs to use the rhashtable-based xattr path and switch from an
> > embedded struct to pointer-based lazy allocation.
> > 
> > Change kernfs_iattrs.xattrs from embedded 'struct simple_xattrs' to a
> > pointer 'struct simple_xattrs *', initialized to NULL (zeroed by
> > kmem_cache_zalloc). Since kernfs_iattrs is already lazily allocated
> > itself, this adds a second level of lazy allocation specifically for
> > the xattr store.
> > 
> > The xattr store is allocated on first setxattr. Read paths
> > check for NULL and return -ENODATA or empty list.
> > 
> > Replaced xattr entries are freed via simple_xattr_free_rcu() to allow
> > concurrent RCU readers to finish.
> > 
> > The cleanup paths in kernfs_free_rcu() and __kernfs_new_node() error
> > handling conditionally free the xattr store only when allocated.
> > 
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> 
> ...
> 
> > @@ -584,6 +582,12 @@ void kernfs_put(struct kernfs_node *kn)
> >  	if (kernfs_type(kn) == KERNFS_LINK)
> >  		kernfs_put(kn->symlink.target_kn);
> >  
> > +	if (kn->iattr && kn->iattr->xattrs) {
> > +		simple_xattrs_free(kn->iattr->xattrs, NULL);
> > +		kfree(kn->iattr->xattrs);
> > +		kn->iattr->xattrs = NULL;
> > +	}
> > +
> >  	spin_lock(&root->kernfs_idr_lock);
> >  	idr_remove(&root->ino_idr, (u32)kernfs_ino(kn));
> >  	spin_unlock(&root->kernfs_idr_lock);
> 
> This is a slight change in the lifetime rules because previously kernfs
> xattrs could be safely accessed only under RCU but after this change you
> have to hold inode reference *and* RCU to safely access them. I don't think
> anybody would be accessing xattrs without holding inode reference so this
> should be safe but it would be good to mention this in the changelog.

Done.