Re: [PATCH V2 4/4] cpuset,mm: update task's mems_allowed lazily

[Miao Xie]

on 2010-3-11 16:15, Nick Piggin wrote:
> On Tue, Mar 09, 2010 at 03:25:54PM +0800, Miao Xie wrote:
>> on 2010-3-9 5:46, David Rientjes wrote:
>> [snip]
>>>> Considering the change of task->mems_allowed is not frequent, so in this patch,
>>>> I use two variables as a tag to indicate whether task->mems_allowed need be
>>>> update or not. And before setting the tag, cpuset caches the new mask of every
>>>> task at its task_struct.
>>>>
>>>
>>> So what exactly is the benefit of 58568d2 from last June that caused this 
>>> issue to begin with?  It seems like this entire patchset is a revert of 
>>> that commit.  So why shouldn't we just revert that one commit and then add 
>>> the locking and updating necessary for configs where
>>> MAX_NUMNODES > BITS_PER_LONG on top?
>>
>> I worried about the consistency of task->mempolicy with task->mems_allowed for
>> configs where MAX_NUMNODES <= BITS_PER_LONG. 
>>
>> The problem that I worried is fowllowing:
>> When the kernel allocator allocates pages for tasks, it will access task->mempolicy
>> first and get the allowed node, then check whether that node is allowed by
>> task->mems_allowed.
>>
>> But, Without this patch, ->mempolicy and ->mems_allowed is not updated at the same
>> time. the kernel allocator may access the inconsistent information of ->mempolicy
>> and ->mems_allowed, sush as the allocator gets the allowed node from old mempolicy,
>> but checks whether that node is allowed by new mems_allowed which does't intersect
>> old mempolicy.
>>
>> So I made this patchset.
> 
> I like your focus on keeping the hotpath light, but it is getting a bit
> crazy. I wonder if it wouldn't be better just to teach those places that
> matter to retry on finding an inconsistent nodemask? The only failure
> case to worry about is getting an empty nodemask, isn't it?
> 

Ok, I try to make a new patch by using seqlock.

Miao

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