The patch titled
Subject: mm: fix anon_vma->degree underflow in anon_vma endless growing prevention
has been added to the -mm tree. Its filename is
mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention.patch
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From: Leon Yu <chianglungyu@xxxxxxxxx>
Subject: 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.
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: David Rientjes <rientjes@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/rmap.c | 7 +++++++
1 file changed, 7 insertions(+)
diff -puN mm/rmap.c~mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention mm/rmap.c
--- a/mm/rmap.c~mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention
+++ a/mm/rmap.c
@@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct
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;
}
_
Patches currently in -mm which might be from chianglungyu@xxxxxxxxx are
mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention.patch
mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention-fix.patch
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