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

[Nick Piggin]

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?

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