linux-kernel - Re: [PATCH] fork: fix oops after fork failure

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Message-ID: <20120823144529.GD19968@dhcp22.suse.cz>
Date:	Thu, 23 Aug 2012 16:45:29 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] fork: fix oops after fork failure

On Thu 23-08-12 16:38:50, Michal Hocko wrote:
> On Thu 23-08-12 16:33:12, Michal Hocko wrote:
> > On Thu 23-08-12 17:08:46, Glauber Costa wrote:
> > > When we want to duplicate a new process, dup_task_struct() will undergo
> > > a series of allocations. If alloc_thread_info_node() fails, we call
> > > free_task_struct() and return.
> > > 
> > > This seems right, but it is not. free_task_struct() will not only free
> > > the task struct from the kmem_cache, but will also call
> > > arch_release_task_struct(). The problem is that this function is
> > > supposed to undo whatever arch-specific work done by
> > > arch_dup_task_struct(), that is not yet called at this point.  The
> > > particular problem I ran accross was that in x86, we will arrive at
> > > fpu_free() without having ever allocated it.
> > > 
> > > This code is very ancient, and according to git, it is there since the
> > > pre-git era. But forks don't fail that often, so that made it well
> > > hidden.
> > > 
> > > Signed-off-by: Glauber Costa <glommer@...allels.com>
> > > Reported-by: Frederic Weisbecker <fweisbec@...hat.com>
> > > ---
> > >  kernel/fork.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index 152d023..b397435 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -299,7 +299,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
> > >  
> > >  	ti = alloc_thread_info_node(tsk, node);
> > >  	if (!ti) {
> > > -		free_task_struct(tsk);
> > > +		kmem_cache_free(task_struct_cachep, tsk);
> > 
> > What about ia64 (or !CONFIG_ARCH_THREAD_INFO_ALLOCATOR in general) which
> > doesn't allocate thread_info at all?
> 
> Hit send button too fast. Should read (or CONFIG_ARCH_THREAD_INFO_ALLOCATOR)
> ia64 will not fail obviously and there is no other arch which would
> define own thread infor allocators but there might be some in future.

Bahh, and I should have been looking at CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
instead. Anyway ia64 uses page allocator directly so kmem_cache_free is
not appropriate.
-- 
Michal Hocko
SUSE Labs
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