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

[Kostya Serebryany]

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

[Richard Henderson (rth)]

Without actually looking, an allocation of 2**42 (4PB) requires
2**30 (1G) pages, and thus 1G page table entries, so 16GB memory
allocation sounds about right for qemu's internal page table allocation.

We need to change data structures for representing guest memory,
probably akin to the kernel's VMAs.

[Stefan Weil (ubuntu-weilnetz)]

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.

[Thomas Huth (th-huth)]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.

[Richard Henderson (rth)]

Still an issue, yes.

[Thomas Huth (th-huth)]

This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'expired' now.
Please continue with the discussion here:

 https://gitlab.com/qemu-project/qemu/-/issues/290