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. > > > 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. > > > 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. > > > 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. > > > 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. > 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... -- Michal Hocko SUSE Labs
