Messages in this thread |  | | Subject | Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap | From | Yang Shi <> | Date | Mon, 6 Aug 2018 09:46:30 -0700 |
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On 8/6/18 2:40 AM, Michal Hocko wrote: > On Fri 03-08-18 14:01:58, Yang Shi wrote: >> >> On 8/3/18 2:07 AM, Michal Hocko wrote: >>> On Fri 27-07-18 02:10:14, Yang Shi 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 >>>> ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0 >>>> ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040 >>>> 00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000 >>>> Call Trace: >>>> [<ffffffff817154d0>] ? __schedule+0x250/0x730 >>>> [<ffffffff817159e6>] schedule+0x36/0x80 >>>> [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150 >>>> [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30 >>>> [<ffffffff81717db0>] down_read+0x20/0x40 >>>> [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0 >>>> [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100 >>>> [<ffffffff81241d87>] __vfs_read+0x37/0x150 >>>> [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0 >>>> [<ffffffff81242266>] vfs_read+0x96/0x130 >>>> [<ffffffff812437b5>] SyS_read+0x55/0xc0 >>>> [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5 >>>> >>>> It is because munmap holds mmap_sem exclusively from very beginning to >>>> all the way down to the end, and doesn't release it in the middle. When >>>> unmapping large mapping, it may take long time (take ~18 seconds to >>>> unmap 320GB mapping with every single page mapped on an idle machine). >>>> >>>> Zapping pages is the most time consuming part, according to the >>>> suggestion from Michal Hocko [1], zapping pages can be done with holding >>>> read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write >>>> mmap_sem to cleanup vmas. >>>> >>>> But, some part may need write mmap_sem, for example, vma splitting. So, >>>> the design is as follows: >>>> acquire write mmap_sem >>>> lookup vmas (find and split vmas) >>>> detach vmas >>>> deal with special mappings >>>> downgrade_write >>>> >>>> zap pages >>>> free page tables >>>> release mmap_sem >>>> >>>> The vm events with read mmap_sem may come in during page zapping, but >>>> since vmas have been detached before, they, i.e. page fault, gup, etc, >>>> will not be able to find valid vma, then just return SIGSEGV or -EFAULT >>>> as expected. >>>> >>>> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are >>>> considered as special mappings. They will be dealt with before zapping >>>> pages with write mmap_sem held. Basically, just update vm_flags. >>> Well, I think it would be safer to simply fallback to the current >>> implementation with these mappings and deal with them on top. This would >>> make potential issues easier to bisect and partial reverts as well. >> Do you mean just call do_munmap()? It sounds ok. Although we may waste some >> cycles to repeat what has done, it sounds not too bad since those special >> mappings should be not very common. > VM_HUGETLB is quite spread. Especially for DB workloads.
Wait a minute. In this way, it sounds we go back to my old implementation with special handling for those mappings with write mmap_sem held, right?
> >>>> And, since they are also manipulated by unmap_single_vma() which is >>>> called by unmap_vma() with read mmap_sem held in this case, to >>>> prevent from updating vm_flags in read critical section, a new >>>> parameter, called "skip_flags" is added to unmap_region(), unmap_vmas() >>>> and unmap_single_vma(). If it is true, then just skip unmap those >>>> special mappings. Currently, the only place which pass true to this >>>> parameter is us. >>> skip parameters are usually ugly and lead to more mess later on. Can we >>> do without them? >> We need a way to tell unmap_region() that it is called in a kind of special >> context which updating vm_flags is not allowed. I didn't think of a better >> way. >> >> We could add a new API to do what unmap_region() does without updating >> vm_flags, but we would have to duplicate some code. > I really didn't get to think about a better way myself but I strongly > suspect we can do without special hacks here. Is updating flags under > read lock a real problem? Assuming that special mappings are not really > considered at this stage.
In normal case, I don't think vm_flags can be updated with read mmap_sem, but in this patch the vmas have been detached from the rb tree, nobody can find them anymore (I'm supposed all vma looking up is done by find_vma), so it might be safe to update vm_flags with read mmap_sem.
If it is safe, we don't have to have any special handling to those special mappings anymore.
> >>>> With this approach we don't have to re-acquire mmap_sem again to clean >>>> up vmas to avoid race window which might get the address space changed. >>> By with this approach you mean detaching right? >> Yes, the detaching approach. > Please make it explicit in the changelog.
Sure.
> >>>> And, since the lock acquire/release cost is managed to the minimum and >>>> almost as same as before, the optimization could be extended to any size >>>> of mapping without incurring significant penalty to small mappings. >>> I guess you mean to say that lock downgrade approach doesn't lead to >>> regressions because the overal time mmap_sem is taken is not longer? >> Yes. And, there is not lock take/retake cost since we don't release it. > Please also be explicit.
Sure.
> >>>> For the time being, just do this in munmap syscall path. Other >>>> vm_munmap() or do_munmap() call sites (i.e mmap, mremap, etc) remain >>>> intact for stability reason. >>> You have used this argument previously and several people have asked. >>> I think it is just wrong. Either the concept is safe and all callers can >>> use it or it is not and then those subtle differences should be called >>> out. Your previous response was that you simply haven't tested other >>> paths. Well, that is not an argument, I am afraid. The whole thing >>> should be done at a proper layer. If there are some difficulties to >>> achieve that for all callers then OK just be explicit about that. I can >>> imagine some callers really require the exclusive look when munmap >>> returns for example. >> Yes, the statement here sounds ambiguous. There are definitely some >> difficulties to achieve that in mmap and mremap. Since they acquire write >> mmap_sem at the very beginning, then do their stuff, which may call >> do_munmap if overlapped address space has to be changed. > Do call them out. Maybe even add a comment in the code so that people > who would like those other paths know what they need to look at.
OK
> >> But, the optimized do_munmap would like to be called without mmap_sem held >> so that we can do the optimization. So, if we want to do the similar >> optimization for mmap/mremap path, I'm afraid we would have to redesign >> them. >> >> I assumes munmap itself is the main source of the latency issue. mmap/mremap >> might hit the latency problem if they are trying to map or remap a huge >> overlapped address space, but it should be rare. So, I leave them untouched. > That depends on usecases very much. mremap might be called on very large > areas as well. But let's go in smaller steps and build on top...
Yes, agree. And, I agree to achieve it step by step.
Thanks, Yang
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