On Thu, Apr 23, 2015 at 10:00 PM, Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> From: Leon Yu <chianglungyu@xxxxxxxxx>
>
> This patch has been added to the 3.18 stable tree. If you have any
> objections, please let us know.
>
> ===============
>
> [ Upstream commit 3fe89b3e2a7bbf3e97657104b9b33a9d81b950b3 ]
As far as I know this's fix for commit which isn't in 3.18 yet
you need to pick these commits before:
7a3ef208e662f4b63d43a23f61a64a129c525bbc
mm: prevent endless growth of anon_vma hierarchy
b800c91a0517071156e772d4fb329ad33590da62
mm: fix corner case in anon_vma endless growing prevention
and then this one
3fe89b3e2a7bbf3e97657104b9b33a9d81b950b3
mm: fix anon_vma->degree underflow in anon_vma endless growing prevention
>
> 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.
>
> Also suggested by Michal Hocko, we can clean up vma_adjust() a bit as
> anon_vma_clone() now does the work.
>
> [akpm@xxxxxxxxxxxxxxxxxxxx: tweak comment]
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Signed-off-by: Leon Yu <chianglungyu@xxxxxxxxx>
> Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> ---
> mm/mmap.c | 2 +-
> mm/rmap.c | 7 +++++++
> 2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 3c83bec..f88b4f9 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -778,10 +778,10 @@ again: remove_next = 1 + (end > next->vm_end);
> if (exporter && exporter->anon_vma && !importer->anon_vma) {
> int error;
>
> + importer->anon_vma = exporter->anon_vma;
> error = anon_vma_clone(importer, exporter);
> if (error)
> return error;
> - importer->anon_vma = exporter->anon_vma;
> }
> }
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 3e4c721..eeafed7 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -261,6 +261,13 @@ 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 callers of anon_vma_clone() don't care
> + * about dst->anon_vma if anon_vma_clone() failed.
> + */
> + dst->anon_vma = NULL;
> unlink_anon_vmas(dst);
> return -ENOMEM;
> }
> --
> 2.1.0
>
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