Re: Very slow unlockall()

[Milan Broz <gmazyland@gmail.com>]

On 01/02/2021 14:08, Vlastimil Babka wrote:
> On 1/8/21 3:39 PM, 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;
>>>> }

...

>> 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
> 
> The perf report would be more interesting from this configuration.

ok, I cannot run perf on that particular VM but tried the latest Fedora stable
kernel without debug options  - 5.10.12-200.fc33.x86_64

This is the report running reproducer above:

time:
real    0m6.123s
user    0m0.099s
sys     0m5.310s

perf:

# Total Lost Samples: 0
#
# Samples: 20K of event 'cycles'
# Event count (approx.): 20397603279
#
# Overhead  Command  Shared Object      Symbol                      
# ........  .......  .................  ............................
#
    47.26%  lock     [kernel.kallsyms]  [k] follow_page_mask
    20.43%  lock     [kernel.kallsyms]  [k] munlock_vma_pages_range
    15.92%  lock     [kernel.kallsyms]  [k] follow_page
     7.40%  lock     [kernel.kallsyms]  [k] rcu_all_qs
     5.87%  lock     [kernel.kallsyms]  [k] _cond_resched
     3.08%  lock     [kernel.kallsyms]  [k] follow_huge_addr
     0.01%  lock     [kernel.kallsyms]  [k] __update_load_avg_cfs_rq
     0.01%  lock     [kernel.kallsyms]  [k] ____fput
     0.01%  lock     [kernel.kallsyms]  [k] rmap_walk_file
     0.00%  lock     [kernel.kallsyms]  [k] page_mapped
     0.00%  lock     [kernel.kallsyms]  [k] native_irq_return_iret
     0.00%  lock     [kernel.kallsyms]  [k] _raw_spin_lock_irq
     0.00%  lock     [kernel.kallsyms]  [k] perf_iterate_ctx
     0.00%  lock     [kernel.kallsyms]  [k] finish_task_switch
     0.00%  perf     [kernel.kallsyms]  [k] native_sched_clock
     0.00%  lock     [kernel.kallsyms]  [k] native_write_msr
     0.00%  perf     [kernel.kallsyms]  [k] native_write_msr


m.