[PATCH] mm: fix corner case in anon_vma endless growing prevention

[Konstantin Khlebnikov]

Fix for BUG_ON(anon_vma->degree) splashes in unlink_anon_vmas()
("kernel BUG at mm/rmap.c:399!").

Anon_vma_clone() is usually called for a copy of source vma in destination
argument. If source vma has anon_vma it should be already in dst->anon_vma.
NULL in dst->anon_vma is used as a sign that it's called from anon_vma_fork().
In this case anon_vma_clone() finds anon_vma for reusing.

Vma_adjust() calls it differently and this breaks anon_vma reusing logic:
anon_vma_clone() links vma to old anon_vma and updates degree counters but
vma_adjust() overrides vma->anon_vma right after that. As a result final
unlink_anon_vmas() decrements degree for wrong anon_vma.

This patch assigns ->anon_vma before calling anon_vma_clone().

Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Tested-by: Chris Clayton <chris2553@xxxxxxxxxxxxxx>
Tested-by: Oded Gabbay <oded.gabbay@xxxxxxx>
Cc: Daniel Forrest <dan.forrest@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
---
 mm/mmap.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 7b36aa7..12616c5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -778,10 +778,12 @@ 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)
+			if (error) {
+				importer->anon_vma = NULL;
 				return error;
-			importer->anon_vma = exporter->anon_vma;
+			}
 		}
 	}
 

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