On Sun, Mar 08, 2009 at 06:07:58PM +0100, Renzo Davoli wrote:
> Eeek! page_mapcount(page) went negative! (-1)
> Then the entire outer uml crashes.
> I have spent the entire day yesterday trying to figure out what has changed
> since
> the beginning of February when it worked.
>
> On 2.6.29-rc7 I get a different error (the Eeek error has been eliminated
> from the
> kernel source code): the output of the nested execution is at the end of this
> message.
> The nested UML crashes anyway, but the calling UML does not panic.
I am hunting the bug...
The following code contains the same mmap64 call that lead nested uml to fail.
-----
#define _GNU_SOURCE
#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
main() {
int fd;
void * addr;
fd = open("/tmp/test",O_CREAT|O_RDWR,0x777);
addr = mmap64((void *) 0x100000, 0x1000,
PROT_EXEC, MAP_FIXED | MAP_PRIVATE, fd, 0x15e000);
printf("%x\n",addr);
munmap(addr,0x1000);
close(fd);
}
----
If you compile and run this code on 2.6.28, UML crashes completely.
On 2.6.29 it returns the same memory errors of a nested UML execution.
mmap64 is a system call.
In the outer UML it is managed by sys_mmap2.
sys_mmap2 calls do_mmap_pgoff
do_mmap_pgoff calls mmap_region
mmap region seems to find the region already allocated, thus it
unmaps the first page:
----
munmap_back:
vma = find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
if (vma && vma->vm_start < addr + len) {
if (do_munmap(mm, addr, len))
return -ENOMEM;
goto munmap_back;
}
----
It fails when returning from the second call of find_vma_prepare.
Is there somebody out there with an enlighting idea on what is going on?
ciao
renzo
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