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


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