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