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

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Dec 21, 2012 at 4:59 PM, Michel Lespinasse <walken@google.com> wrote:
> On Fri, Dec 21, 2012 at 4:36 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Thu, Dec 20, 2012 at 4:49 PM, Michel Lespinasse <walken@google.com> 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 :/

Hmm.  I'm using MCL_FUTURE with MAP_NORESERVE, but those mappings are
not insanely large.  Should MAP_NORESERVE would negate MCL_FUTURE?
I'm doing MAP_NORESERVE, PROT_NONE to prevent pages from being
allocated in the future -- I have no intention of ever using them.

The other odd thing I do is use MAP_FIXED to replace MAP_NORESERVE pages.

--Andy

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