at 12:18 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > On Tue 19-06-18 17:31:27, Nadav Amit wrote: >> at 4:08 PM, Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote: >> >>> On 6/19/18 3:17 PM, Nadav Amit wrote: >>>> at 4:34 PM, Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> >>>> wrote: >>>> >>>> >>>>> When running some mmap/munmap scalability tests with large memory (i.e. >>>>> >>>>>> 300GB), the below hung task issue may happen occasionally. >>>>> INFO: task ps:14018 blocked for more than 120 seconds. >>>>> Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1 >>>>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this >>>>> message. >>>>> ps D 0 14018 1 0x00000004 >>>> (snip) >>>> >>>> >>>>> Zapping pages is the most time consuming part, according to the >>>>> suggestion from Michal Hock [1], zapping pages can be done with holding >>>>> read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write >>>>> mmap_sem to manipulate vmas. >>>> Does munmap() == MADV_DONTNEED + munmap() ? >>> >>> Not exactly the same. So, I basically copied the page zapping used by munmap instead of calling MADV_DONTNEED. >>> >>>> For example, what happens with userfaultfd in this case? Can you get an >>>> extra #PF, which would be visible to userspace, before the munmap is >>>> finished? >>> >>> userfaultfd is handled by regular munmap path. So, no change to userfaultfd part. >> >> Right. I see it now. >> >>>> In addition, would it be ok for the user to potentially get a zeroed page in >>>> the time window after the MADV_DONTNEED finished removing a PTE and before >>>> the munmap() is done? >>> >>> This should be undefined behavior according to Michal. This has been discussed in https://lwn.net/Articles/753269/. >> >> Thanks for the reference. >> >> Reading the man page I see: "All pages containing a part of the indicated >> range are unmapped, and subsequent references to these pages will generate >> SIGSEGV.” > > Yes, this is true but I guess what Yang Shi meant was that an userspace > access racing with munmap is not well defined. You never know whether > you get your data, #PTF or SEGV because it depends on timing. The user > visible change might be that you lose content and get zero page instead > if you hit the race window while we are unmapping which was not possible > before. But whouldn't such an access pattern be buggy anyway? You need > some form of external synchronization AFAICS. It seems to follow the specifications, so it is not clearly buggy IMHO. I don’t know of such a use-case, but if somebody does so - the proposed change might even cause a security vulnerability. > But maybe some userspace depends on "getting right data or get SEGV" > semantic. If we have to preserve that then we can come up with a VM_DEAD > flag set before we tear it down and force the SEGV on the #PF path. > Something similar we already do for MMF_UNSTABLE. That seems reasonable. Regards, Nadav
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