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
