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@linux-foundation.org: tweak comment]
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: Leon Yu <chianglungyu@gmail.com>
Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index da9990a..9ec50a3 100644 (file)
--- 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;
-                       }
                }
        }
 

diff --git a/mm/rmap.c b/mm/rmap.c
index 5e3e090..c161a14 100644 (file)
--- 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 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;
 }