From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vladimir Davydov Subject: Re: [PATCH v12 05/18] fs: do not use destroy_super() in alloc_super() fail path Date: Tue, 3 Dec 2013 13:23:01 +0400 Message-ID: <529DA2F5.1040602@parallels.com> References: <20131203090041.GB8803@dastard> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20131203090041.GB8803@dastard> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Dave Chinner Cc: hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, mhocko-AlSwsSmVLrQ@public.gmane.org, dchinner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, devel-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org, glommer-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org, Al Viro On 12/03/2013 01:00 PM, Dave Chinner wrote: > On Mon, Dec 02, 2013 at 03:19:40PM +0400, Vladimir Davydov wrote: >> Using destroy_super() in alloc_super() fail path is bad, because: >> >> * It will trigger WARN_ON(!list_empty(&s->s_mounts)) since s_mounts is >> initialized after several 'goto fail's. > So let's fix that. > >> * It will call kfree_rcu() to free the super block although kfree() is >> obviously enough there. >> * The list_lru structure was initially implemented without the ability >> to destroy an uninitialized object in mind. >> >> I'm going to replace the conventional list_lru with per-memcg lru to >> implement per-memcg slab reclaim. This new structure will fail >> destruction of objects that haven't been properly initialized so let's >> inline appropriate snippets from destroy_super() to alloc_super() fail >> path instead of using the whole function there. > You're basically undoing the change made in commit 7eb5e88 ("uninline > destroy_super(), consolidate alloc_super()") which was done less > than a month ago. :/ > > The code as it stands works just fine - the list-lru structures in > the superblock are actually initialised (to zeros) - and so calling > list_lru_destroy() on it works just fine in that state as the > pointers that are freed are NULL. Yes, unexpected, but perfectly > valid code. > > I haven't looked at the internals of the list_lru changes you've > made yet, but it surprises me that we can't handle this case > internally to list_lru_destroy(). Actually, I'm not going to modify the list_lru structure, because I think it's good as it is. I'd like to substitute it with a new structure, memcg_list_lru, only in those places where this functionality (per-memcg scanning) is really needed. This new structure would look like this: struct memcg_list_lru { struct list_lru global_lru; struct list_lru **memcg_lrus; struct list_head list; void *old_lrus; } Since old_lrus and memcg_lrus can be NULL under normal operation, in memcg_list_lru_destroy() I'd have to check either the list or the global_lru field, i.e. it would look like: if (!list.next) /* has not been initialized */ return; or if (!global_lru.node) /* has not been initialized */ return; I find both of these checks ugly :-( Personally, I think that's calling destroy() w/o init() is OK only for simple structures where destroy/init are inline functions or macros, otherwise one can forget to "fix" destroy() after it extends a structure. Thanks.