Re: [PATCH v2] mm/hugetlb: defer free_huge_page() to a workqueue

[Mike Kravetz]

On 12/16/19 8:17 AM, Davidlohr Bueso wrote:
> On Mon, 16 Dec 2019, Michal Hocko wrote:
>> I am afraid that work_struct is too large to be stuffed into the struct
>> page array (because of the lockdep part).
> 
> Yeah, this needs to be done without touching struct page.
> 
> Which is why I had done the stack allocated way in this patch, but we
> cannot wait for it to complete in irq, so that's out the window. Andi
> had suggested percpu allocated work items, but having played with the
> idea over the weekend, I don't see how we can prevent another page being
> freed on the same cpu before previous work on the same cpu is complete
> (cpu0 wants to free pageA, schedules the work, in the mean time cpu0
> wants to free pageB and workerfn for pageA still hasn't been called).
> 
>> I think that it would be just safer to make hugetlb_lock irq safe. Are
>> there any other locks that would require the same?
> 
> It would be simpler. Any performance issues that arise would probably
> be only seen in microbenchmarks, assuming we want to have full irq safety.
> If we don't need to worry about hardirq, then even better.
> 
> The subpool lock would also need to be irq safe.

I do think we need to worry about hardirq.  There are no restruictions that
put_page can not be called from hardirq context. 

I am concerned about the latency of making hugetlb_lock (and potentially
subpool lock) hardirq safe.  When these locks were introduced (before my
time) the concept of making them irq safe was not considered.  Recently,
I learned that the hugetlb_lock is held for a linear scan of ALL hugetlb
pages during a cgroup reparentling operation.  That is just too long.

If there is no viable work queue solution, then I think we would like to
restructure the hugetlb locking before a change to just make hugetlb_lock
irq safe.  The idea would be to split the scope of what is done under
hugetlb_lock.  Most of it would never be executed in irq context.  Then
have a small/limited set of functionality that really needs to be irq
safe protected by an irq safe lock.

-- 
Mike Kravetz