[CRIU] [PATCH] mem: fix typo in determining an address of parent vma

[Filipe Brandenburger]

It turns out that when you apply this patch then it will actually COW
too much...

For instance, the sep_tcs test case of cow01 fails for me with this patch in.

The reason is that the vmas on parent and child have same start and
end but are not really COWed from one another.

The problem is that the zero pages on the child not returned by
pr.get_pagemap() are not handled by restore_priv_vma_content() so they
end up with the contents of the parent instead of zeroed out contents.
(There seems to be a similar problem with the file private mmap in
file_tcs as well but I didn't go far enough in troubleshooting that
one yet.)

I'd suggest holding on this patch until those other issues are
addressed, otherwise you'll go from having not COWed pages with the
right memory contents to having pages with the wrong contents in them
which is worse.

I think the patches to fix traversal of COW vmas belong before this
one on the order of patches.

Cheers,
Filipe


On Tue, May 13, 2014 at 4:39 AM, Andrey Vagin <avagin at openvz.org> wrote:
> Look at this hunk from 7659c995f58f:
> -    paddr = decode_pointer(vma_premmaped_start(&p->vma));
> +    paddr = decode_pointer(vma->premmaped_addr);
>
> Obviously we want to use p->premmaped_addr instead of
> vma->premmaped_addr.
>
> Fixes: 7659c995f58f ("vm: don't overwrite vma->shmid for private mappings")
> Reported-by: Filipe Brandenburger <filbranden at google.com>
> Signed-off-by: Andrey Vagin <avagin at openvz.org>
> ---
>  cr-restore.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/cr-restore.c b/cr-restore.c
> index d79da59..897c3fc 100644
> --- a/cr-restore.c
> +++ b/cr-restore.c
> @@ -260,7 +260,7 @@ static int map_private_vma(pid_t pid, struct vma_area *vma, void **tgt_addr,
>
>                 pr_info("COW 0x%016"PRIx64"-0x%016"PRIx64" 0x%016"PRIx64" vma\n",
>                         vma->e->start, vma->e->end, vma->e->pgoff);
> -               paddr = decode_pointer(vma->premmaped_addr);
> +               paddr = decode_pointer(p->premmaped_addr);
>         }
>
>         *pvma = p;
> --
> 1.8.5.3
>