Messages in this thread | | | Date | Fri, 21 Dec 2012 17:59:54 -0800 | Subject | Re: [PATCH 0/9] Avoid populating unbounded num of ptes with mmap_sem held | From | Michel Lespinasse <> |
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On Fri, Dec 21, 2012 at 5:09 PM, Andy Lutomirski <luto@amacapital.net> wrote: > 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: >>> 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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