Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping

[Yang Shi <yang.shi@linux.alibaba.com>]

On 7/2/18 6:53 AM, Michal Hocko wrote:
> On Sat 30-06-18 06:39:44, 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 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).
>>
>> It is because munmap holds mmap_sem 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 Hock [1], zapping pages can be done with holding
> s@Hock@Hocko@

Sorry for the wrong spelling.

>
>> read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write
>> mmap_sem to cleanup vmas. All zapped vmas will have VM_DEAD flag set,
>> the page fault to VM_DEAD vma will trigger SIGSEGV.
> This really deserves an explanation why the all dance is really needed.
>
> It would be also good to mention how do you achieve the overal
> consistency. E.g. you are dropping mmap_sem and then re-taking it for
> write. What if any pending write lock succeeds and modify the address
> space? Does it matter, why if not?

Sure.

>
>> Define large mapping size thresh as PUD size or 1GB, just zap pages with
>> read mmap_sem for mappings which are >= thresh value.
>>
>> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, then just
>> fallback to regular path since unmapping those mappings need acquire
>> write mmap_sem.
>>
>> For the time being, just do this in munmap syscall path. Other
>> vm_munmap() or do_munmap() call sites remain intact for stability
>> reason.
> What are those stability reasons?

mmap() and mremap() may call do_munmap() as well, so it may introduce 
more race condition if they use the zap early version of do_munmap too. 
They would have much more chances to take mmap_sem to change address 
space and cause conflict.

And, it looks they are not the vital source of long period of write 
mmap_sem hold. So, it sounds not worth making things more complicated 
for the time being.

>
>> The below is some regression and performance data collected on a machine
>> with 32 cores of E5-2680 @ 2.70GHz and 384GB memory.
>>
>> With the patched kernel, write mmap_sem hold time is dropped to us level
>> from second.
> I haven't read through the implemenation carefuly TBH but the changelog
> needs quite some work to explain the solution and resulting semantic of
> munmap after the change.

Thanks for the suggestion. Will polish the changelog.

Yang