Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap

[Yang Shi]

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.

> > 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.

> > 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.

> > 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.

> > 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.

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.

> > With the patches, exclusive mmap_sem hold time when munmap a 80GB
> > address space on a machine with 32 cores of E5-2680 @ 2.70GHz dropped to
> > us level from second.
> >
> > munmap_test-15002 [008]   594.380138: funcgraph_entry: |  vm_munmap_zap_rlock() {
> > munmap_test-15002 [008]   594.380146: funcgraph_entry:      !2485684 us |    unmap_region();
> > munmap_test-15002 [008]   596.865836: funcgraph_exit:       !2485692 us |  }
> >
> > Here the excution time of unmap_region() is used to evaluate the time of
> > holding read mmap_sem, then the remaining time is used with holding
> > exclusive lock.
> I will be reading through the patch and follow up on that separately.

Thanks,
Yang