Re: [PATCH 1/2] mm: fix bugs of mpol_rebind_nodemask()

[Miao Xie <miaox@cn.fujitsu.com>]

on 2010-4-30 2:03, David Rientjes wrote:
> 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. ]

I updated it and remade a new patchset, could you review it for me?

Thanks
Miao