qemu-x86_64 runs out of memory

qemu-x86_64 runs out of memory

[Andreas Schwab]

$ cat mmap.c
#include <stdio.h>
#include <sys/mman.h>

int
main (void)
{
  void *A;
  size_t L = 0, U, Max = 0;
  for (U = 1; ; U *= 2)
    {
      A = mmap (0, U, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
      if (A == (void *) -1)
	break;
      else
	munmap (A, U);
    }
  while (L + 1 < U)
    {
      size_t M = L + (U - L) / 2;
      A = mmap (0, M, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
      if (A == (void *) -1)
	U = M;
      else
	{
	  Max = M;
	  munmap(A, M);
	  L = M;
	}
    }
  printf ("%zx\n", Max);
}
$ cc -O2 -g -Wall -std=gnu11 -g     mmap.c   -o mmap
$ ./mmap 
7f672e5ff000
$ qemu-x86_64 ./mmap
Killed

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."

Re: qemu-x86_64 runs out of memory

[Richard Henderson]

On 9/13/22 10:35, Andreas Schwab wrote:
> $ cat mmap.c
> #include <stdio.h>
> #include <sys/mman.h>
> 
> int
> main (void)
> {
>    void *A;
>    size_t L = 0, U, Max = 0;
>    for (U = 1; ; U *= 2)
>      {
>        A = mmap (0, U, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
>        if (A == (void *) -1)
> 	break;
>        else
> 	munmap (A, U);
>      }
>    while (L + 1 < U)
>      {
>        size_t M = L + (U - L) / 2;
>        A = mmap (0, M, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
>        if (A == (void *) -1)
> 	U = M;
>        else
> 	{
> 	  Max = M;
> 	  munmap(A, M);
> 	  L = M;
> 	}
>      }
>    printf ("%zx\n", Max);
> }
> $ cc -O2 -g -Wall -std=gnu11 -g     mmap.c   -o mmap
> $ ./mmap
> 7f672e5ff000
> $ qemu-x86_64 ./mmap
> Killed
> 

I suspect this is the same as

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

Needs a reorg of memory region tracking for {linux,bsd}-user.


r~

Re: qemu-x86_64 runs out of memory

[Li Zhang]

I did some investigation and when I mmap 2TB memory, the PageDesc is
allocated with 12GB.
The count of PageDesc is about  0x80000 and the size of PageDesc is 24Bytes.
So memory costs about 12GB.  If the size of PageDesc data is reduced,
the memory should be reduced significantly.

If more and more memory is mapped, and the page table entrieds is very
huge. So it still causes OOM if the host memory is not big enough.


On Tue, Sep 13, 2022 at 12:59 PM Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 9/13/22 10:35, Andreas Schwab wrote:
> > $ cat mmap.c
> > #include <stdio.h>
> > #include <sys/mman.h>
> >
> > int
> > main (void)
> > {
> >    void *A;
> >    size_t L = 0, U, Max = 0;
> >    for (U = 1; ; U *= 2)
> >      {
> >        A = mmap (0, U, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
> >        if (A == (void *) -1)
> >       break;
> >        else
> >       munmap (A, U);
> >      }
> >    while (L + 1 < U)
> >      {
> >        size_t M = L + (U - L) / 2;
> >        A = mmap (0, M, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
> >        if (A == (void *) -1)
> >       U = M;
> >        else
> >       {
> >         Max = M;
> >         munmap(A, M);
> >         L = M;
> >       }
> >      }
> >    printf ("%zx\n", Max);
> > }
> > $ cc -O2 -g -Wall -std=gnu11 -g     mmap.c   -o mmap
> > $ ./mmap
> > 7f672e5ff000
> > $ qemu-x86_64 ./mmap
> > Killed
> >
>
> I suspect this is the same as
>
>    https://gitlab.com/qemu-project/qemu/-/issues/290
>
> Needs a reorg of memory region tracking for {linux,bsd}-user.
>
>
> r~
>


-- 

Best Regards
-Li

Re: qemu-x86_64 runs out of memory

[Alex Bennée]

Andreas Schwab <schwab@suse.de> writes:

> $ cat mmap.c
> #include <stdio.h>
> #include <sys/mman.h>
>
> int
> main (void)
> {
>   void *A;
>   size_t L = 0, U, Max = 0;
>   for (U = 1; ; U *= 2)
>     {
>       A = mmap (0, U, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
>       if (A == (void *) -1)
> 	break;
>       else
> 	munmap (A, U);
>     }
>   while (L + 1 < U)
>     {
>       size_t M = L + (U - L) / 2;
>       A = mmap (0, M, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
>       if (A == (void *) -1)
> 	U = M;
>       else
> 	{
> 	  Max = M;
> 	  munmap(A, M);
> 	  L = M;
> 	}
>     }
>   printf ("%zx\n", Max);
> }
> $ cc -O2 -g -Wall -std=gnu11 -g     mmap.c   -o mmap
> $ ./mmap 
> 7f672e5ff000
> $ qemu-x86_64 ./mmap
> Killed

This is probably the same as bug: https://gitlab.com/qemu-project/qemu/-/issues/967

-- 
Alex Bennée