[Bug 1898011] Re: mmap MAP_NORESERVE of 2^42 bytes consumes 16Gb of actual RAM

[Stefan Weil <1898011@bugs.launchpad.net>]

The problem occurs for example with any program which was compiled with
the address sanitizer.

A simple hello program compiled with "gcc -fsanitize=address hello.c" is
sufficient to show the problem. Just run it with "qemu-x86_64 a.out". It
will be killed by the Linux kernel OOM handler even on a server with 64
GB RAM.

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https://bugs.launchpad.net/bugs/1898011

Title:
  mmap MAP_NORESERVE of 2^42 bytes consumes 16Gb of actual RAM

Status in QEMU:
  Confirmed

Bug description:
  Run this program:

  #include <sys/mman.h>
  #include <stdio.h>
  int main() {
          for (int i = 30; i <= 44; i++) {
                  fprintf(stderr, "trying 2**%d\n", i);
                  mmap((void*)0x600000000000,1ULL << i,
                          PROT_NONE,
                          MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0);
          }
  }

  (tried qemu-x86_64 and qemu-aarch64, 4.2.1 and trunk/5.1.50)

  On each iteration qemu will consume 2x more physical RAM, 
  e.g. when mapping 2^42 it will have RSS of 16Gb.

  On normal linux it works w/o consuming much RAM, due to MAP_NORESERVE.

  Also: qemu -strace prints 0 instead of the correct size starting from size=2^32
  and prints -2147483648 for size=2^31. 

  mmap(0x0000600000000000,1073741824,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0)
  = 0x0000600000000000

  mmap(0x0000600000000000,-2147483648,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0)
  = 0x0000600000000000

  mmap(0x0000600000000000,0,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0)
  = 0x0000600000000000

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