linux-kernel - Re: [PATCH v12 05/18] fs: do not use destroy_super() in alloc

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Message-ID: <20131203090041.GB8803@dastard>
Date:	Tue, 3 Dec 2013 20:00:41 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	hannes@...xchg.org, mhocko@...e.cz, dchinner@...hat.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, cgroups@...r.kernel.org, devel@...nvz.org,
	glommer@...nvz.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v12 05/18] fs: do not use destroy_super() in
 alloc_super() fail path

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().

Al, your call on inlining destroy_super() in alloc_super() again....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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