mm: fix corner case in anon_vma endless growing prevention

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 <[email protected]> Fixes:

7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")

Tested-by Chris Clayton <[email protected]> Tested-by: Oded

Gabbay <[email protected]> Cc: Daniel Forrest <[email protected]> Cc: Michal Hocko <[email protected]> Cc:
Rik van Riel <[email protected]>

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 <[email protected]>

Fixes 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")

Tested-by Chris Clayton <[email protected]>

Tested-by Oded Gabbay <[email protected]>

Cc Daniel Forrest <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Rik van Riel <[email protected]>

---
 mm/mmap.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

mm/mmap.c +4 -3

index 7b36aa7..12616c5 100644
@@ -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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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 <[email protected]>

Fixes 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")

Tested-by Chris Clayton <[email protected]>

Tested-by Oded Gabbay <[email protected]>

Cc Daniel Forrest <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Rik van Riel <[email protected]>

---
 mm/mmap.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

mm/mmap.c +4 -2

index 7b36aa7..12616c5 100644
@@ -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;
+			}
 		}
 	}
 

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 <[email protected]>

Fixes 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")

Tested-by Chris Clayton <[email protected]>

Tested-by Oded Gabbay <[email protected]>

Cc Daniel Forrest <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Rik van Riel <[email protected]>

Quoted 13 lines · On Sun, 11 Jan 2015 16:54:06 +0300 Konstantin Khlebnikov <[email protected]>…

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.

Quoted 4 lines

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

Signed-off-by Konstantin Khlebnikov <[email protected]>

Fixes 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")

I've asked Greg not to take 7a3ef208e662 into -stable because of this problem. So if you still think we should fix this in -stable, could you please prepare an updated patch and send it to Greg?

Quoted 6 lines

Tested-by Chris Clayton <[email protected]>

Tested-by Oded Gabbay <[email protected]>

Cc Daniel Forrest <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Rik van Riel <[email protected]>