On Fri, Dec 21, 2012 at 5:09 PM, Andy Lutomirski [off-list ref] wrote:

On Fri, Dec 21, 2012 at 4:59 PM, Michel Lespinasse [off-list ref] wrote:

quoted

On Fri, Dec 21, 2012 at 4:36 PM, Andy Lutomirski [off-list ref] wrote:

quoted

Something's buggy here.  My evil test case is stuck with lots of
threads spinning at 100% system time.

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 :/

So in my test case, the issue was caused by the mapping being 2^32
pages, which overflowed the integer 'nr_pages' argument to
__get_user_pages, which caused an infinite loop as __get_user_pages()
would return 0 so __mm_populate() would make no progress.

When dropping one zero from that humongous size in the test case, the
test case becomes at least killable.

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.

MAP_NORESERVE doesn't prevent page allocation, but PROT_NONE does
(precisely because people use it the same way as you do :)

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

Yes, I've seen people do that here too.

Could you share your test case so I can try reproducing the issue
you're seeing ?

Thanks,

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

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