linux-kernel - Re: [PATCH 1/2] mm: fix bugs of mpol_rebind

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Date:	Thu, 29 Apr 2010 11:03:21 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Miao Xie <miaox@...fujitsu.com>
cc:	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>, Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: fix bugs of mpol_rebind_nodemask()

On Thu, 29 Apr 2010, Miao Xie wrote:

> > That's been the behavior for at least three years so changing it from 
> > under the applications isn't acceptable, see 
> > Documentation/vm/numa_memory_policy.txt regarding mempolicy rebinds and 
> > the two flags that are defined that can be used to adjust the behavior.
> 
> Is the flags what you said MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES? 
> But the codes that I changed isn't under MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES.
> The documentation doesn't say what we should do if either of these two flags is not set. 
> 

MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES allow you to adjust the 
behavior of the rebind: the former requires specific nodes to be assigned 
to the mempolicy and could suppress the rebind completely, if necessary; 
the latter ensures the mempolicy nodemask has a certain weight as nodes 
are assigned in a round-robin manner.  The behavior that you're referring 
to is provided via MPOL_F_RELATIVE_NODES, which guarantees whatever weight 
is passed via set_mempolicy() will be preserved when mems are added to a 
cpuset.

Regardless of whether the behavior is documented when either flag is 
passed, we can't change the long-standing default behavior that people use 
when their cpuset mems are rebound: we can only extend the functionality 
and the behavior you're seeking is already available with a 
MPOL_F_RELATIVE_NODES flag modifier.

> Furthermore, in order to fix no node to alloc memory, when we want to update mempolicy
> and mems_allowed, we expand the set of nodes first (set all the newly nodes) and
> shrink the set of nodes lazily(clean disallowed nodes).

That's a cpuset implementation choice, not a mempolicy one; mempolicies 
have nothing to do with an empty current->mems_allowed.

> But remap() breaks the expanding, so if we don't remove remap(), the problem can't be
> fixed. Otherwise, cpuset has to do the rebinding by itself and the code is ugly.
> Like this:
> 
> static void cpuset_change_task_nodemask(struct task_struct *tsk, nodemask_t *newmems)
> {
> 	nodemask_t tmp;
> 	...
> 	/* expand the set of nodes */
> 	if (!mpol_store_user_nodemask(tsk->mempolicy)) {
> 		nodes_remap(tmp, ...);
> 		nodes_or(tsk->mempolicy->v.nodes, tsk->mempolicy->v.nodes, tmp);
> 	}
> 	...
> 
> 	/* shrink the set of nodes */
> 	if (!mpol_store_user_nodemask(tsk->mempolicy))
> 		tsk->mempolicy->v.nodes = tmp;
> }
> 

I don't see why this is even necessary, the mempolicy code could simply 
return numa_node_id() when nodes_empty(current->mempolicy->v.nodes) to 
close the race.

 [ Your pseudo-code is also lacking task_lock(tsk), which is required to 
   safely dereference tsk->mempolicy, and this is only available so far in 
   -mm since the oom killer rewrite. ]
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