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SubjectRe: Very slow unlockall()
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On 08/01/2021 15:39, Milan Broz wrote:
> On 08/01/2021 14:41, Michal Hocko wrote:
>> On Wed 06-01-21 16:20:15, Milan Broz wrote:
>>> Hi,
>>>
>>> we use mlockall(MCL_CURRENT | MCL_FUTURE) / munlockall() in cryptsetup code
>>> and someone tried to use it with hardened memory allocator library.
>>>
>>> Execution time was increased to extreme (minutes) and as we found, the problem
>>> is in munlockall().
>>>
>>> Here is a plain reproducer for the core without any external code - it takes
>>> unlocking on Fedora rawhide kernel more than 30 seconds!
>>> I can reproduce it on 5.10 kernels and Linus' git.
>>>
>>> The reproducer below tries to mmap large amount memory with PROT_NONE (later never used).
>>> The real code of course does something more useful but the problem is the same.
>>>
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> #include <fcntl.h>
>>> #include <sys/mman.h>
>>>
>>> int main (int argc, char *argv[])
>>> {
>>> void *p = mmap(NULL, 1UL << 41, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>>>
>>> if (p == MAP_FAILED) return 1;
>>>
>>> if (mlockall(MCL_CURRENT | MCL_FUTURE)) return 1;
>>> printf("locked\n");
>>>
>>> if (munlockall()) return 1;
>>> printf("unlocked\n");
>>>
>>> return 0;
>>> }
>>>
>>> In traceback I see that time is spent in munlock_vma_pages_range.
>>>
>>> [ 2962.006813] Call Trace:
>>> [ 2962.006814] ? munlock_vma_pages_range+0xe7/0x4b0
>>> [ 2962.006814] ? vma_merge+0xf3/0x3c0
>>> [ 2962.006815] ? mlock_fixup+0x111/0x190
>>> [ 2962.006815] ? apply_mlockall_flags+0xa7/0x110
>>> [ 2962.006816] ? __do_sys_munlockall+0x2e/0x60
>>> [ 2962.006816] ? do_syscall_64+0x33/0x40
>>> ...
>>>
>>> Or with perf, I see
>>>
>>> # Overhead Command Shared Object Symbol
>>> # ........ ....... ................. .....................................
>>> #
>>> 48.18% lock [kernel.kallsyms] [k] lock_is_held_type
>>> 11.67% lock [kernel.kallsyms] [k] ___might_sleep
>>> 10.65% lock [kernel.kallsyms] [k] follow_page_mask
>>> 9.17% lock [kernel.kallsyms] [k] debug_lockdep_rcu_enabled
>>> 6.73% lock [kernel.kallsyms] [k] munlock_vma_pages_range
>>> ...
>>>
>>>
>>> Could please anyone check what's wrong here with the memory locking code?
>>> Running it on my notebook I can effectively DoS the system :)
>>>
>>> Original report is https://gitlab.com/cryptsetup/cryptsetup/-/issues/617
>>> but this is apparently a kernel issue, just amplified by usage of munlockall().
>>
>> Which kernel version do you see this with? Have older releases worked
>> better?
>
> Hi,
>
> I tried 5.10 stable and randomly few kernels I have built on testing VM (5.3 was the oldest),
> it seems to be very similar run time, so the problem is apparently old...(I can test some specific kernel version if it make any sense).
>
> For mainline (reproducer above):
>
> With 5.11.0-0.rc2.20210106git36bbbd0e234d.117.fc34.x86_64 (latest Fedora rawhide kernel build - many debug options are on)
>
> # time ./lock
> locked
> unlocked
>
> real 0m32.287s
> user 0m0.001s
> sys 0m32.126s
>
>
> Today's Linus git - 5.11.0-rc2+ in my testing x86_64 VM (no extensive kernel debug options):
>
> # time ./lock
> locked
> unlocked
>
> real 0m4.172s
> user 0m0.000s
> sys 0m4.172s
>
> m.

Hi,

so because there is no response, is this expected behavior of memory management subsystem then?

Thanks,
Milan





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