From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752015Ab3LCJXI (ORCPT ); Tue, 3 Dec 2013 04:23:08 -0500 Received: from relay.parallels.com ([195.214.232.42]:47643 "EHLO relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751252Ab3LCJXE (ORCPT ); Tue, 3 Dec 2013 04:23:04 -0500 Message-ID: <529DA2F5.1040602@parallels.com> Date: Tue, 3 Dec 2013 13:23:01 +0400 From: Vladimir Davydov User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130922 Icedove/17.0.9 MIME-Version: 1.0 To: Dave Chinner CC: , , , , , , , , , Al Viro Subject: Re: [PATCH v12 05/18] fs: do not use destroy_super() in alloc_super() fail path References: <20131203090041.GB8803@dastard> In-Reply-To: <20131203090041.GB8803@dastard> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.30.16.96] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.