Commit 3fe89b3e authored by Leon Yu's avatar Leon Yu Committed by Linus Torvalds
Browse files

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 7a3ef208 ("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.

[ tweak comment]
Fixes: 7a3ef208

 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: default avatarLeon Yu <>
Signed-off-by: default avatarKonstantin Khlebnikov <>
Reviewed-by: default avatarMichal Hocko <>
Acked-by: default avatarRik van Riel <>
Acked-by: default avatarDavid Rientjes <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent ddd2a30d
......@@ -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;
......@@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
return 0;
* 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;
return -ENOMEM;
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