On Tue 03-03-15 20:02:15, Leon Yu wrote:
> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
> to 3.19 and had no luck with 4.0-rc1 neither.
>
> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
> prevent endless growth of anon_vma hierarchy"), I found chances are that
> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
> and eventually underflow when exiting as a result of another call to
> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
> for me.
>
> This patch fixes the underflow by dropping dst->anon_vma when allocation
> fails. It's safe to do so regardless of original value of dst->anon_vma
> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
> Besides, callers don't care dst->anon_vma in such case neither.
>
> Signed-off-by: Leon Yu <chianglungyu@xxxxxxxxx>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@xxxxxxxxxxxxxxx # v3.19
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
I think we can safely remove the following code as well, because it is
anon_vma_clone which is responsible to do all the cleanups.
diff --git a/mm/mmap.c b/mm/mmap.c
index 943c6ad18b1d..06a6076c92e5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -774,10 +774,8 @@ again: remove_next = 1 + (end > next->vm_end);
importer->anon_vma = exporter->anon_vma;
error = anon_vma_clone(importer, exporter);
- if (error) {
- importer->anon_vma = NULL;
+ if (error)
return error;
- }
}
}
> ---
> mm/rmap.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5e3e090..bed3cf2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> return 0;
>
> enomem_failure:
> + /*
> + * dst->anon_vma is dropped here otherwise its degree can be incorrectly
> + * decremented in unlink_anon_vmas().
> + * We can safely do this because calllers of anon_vma_clone() wouldn't
> + * care dst->anon_vma if anon_vma_clone() failed.
> + */
> + dst->anon_vma = NULL;
> unlink_anon_vmas(dst);
> return -ENOMEM;
> }
> --
> 2.3.1
>
--
Michal Hocko
SUSE Labs
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