Re: Very slow unlockall()

[Michal Hocko]

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?
-- 
Michal Hocko
SUSE Labs