Re: [PATCH 0/9] Avoid populating unbounded num of ptes with mmap_sem held

[Michel Lespinasse]

On Fri, Dec 21, 2012 at 4:36 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, Dec 20, 2012 at 4:49 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>> We have many vma manipulation functions that are fast in the typical case,
>> but can optionally be instructed to populate an unbounded number of ptes
>> within the region they work on:
>> - mmap with MAP_POPULATE or MAP_LOCKED flags;
>> - remap_file_pages() with MAP_NONBLOCK not set or when working on a
>>   VM_LOCKED vma;
>> - mmap_region() and all its wrappers when mlock(MCL_FUTURE) is in effect;
>> - brk() when mlock(MCL_FUTURE) is in effect.
>>
>
> Something's buggy here.  My evil test case is stuck with lots of
> threads spinning at 100% system time.  Stack traces look like:
>
> [<0000000000000000>] __mlock_vma_pages_range+0x66/0x70
> [<0000000000000000>] __mm_populate+0xf9/0x150
> [<0000000000000000>] vm_mmap_pgoff+0x9f/0xc0
> [<0000000000000000>] sys_mmap_pgoff+0x7e/0x150
> [<0000000000000000>] sys_mmap+0x22/0x30
> [<0000000000000000>] system_call_fastpath+0x16/0x1b
> [<0000000000000000>] 0xffffffffffffffff
>
> perf top says:
>
>  38.45%  [kernel]            [k] __mlock_vma_pages_range
>  33.04%  [kernel]            [k] __get_user_pages
>  28.18%  [kernel]            [k] __mm_populate
>
> The tasks in question use MCL_FUTURE but not MAP_POPULATE.  These
> tasks are immune to SIGKILL.

Looking into it.

There seems to be a problem with mlockall - the following program
fails in an unkillable way even before my changes:

#include <sys/mman.h>
#include <stdio.h>
#include <stdint.h>

int main(void) {
  void *p = mmap(NULL, 0x100000000000,
                 PROT_READ | PROT_WRITE,
                 MAP_PRIVATE | MAP_ANON | MAP_NORESERVE,
                 -1, 0);
  printf("p: %p\n", p);
  mlockall(MCL_CURRENT);
  return 0;
}

I think my changes propagate this existing problem so it now shows up
in more places :/

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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