ksm: hold anon_vma in rmap_item

For full functionality, page_referenced_one() and try_to_unmap_one() need
to know the vma: to pass vma down to arch-dependent flushes, or to observe
VM_LOCKED or VM_EXEC.  But KSM keeps no record of vma: nor can it, since
vmas get split and merged without its knowledge.

Instead, note page's anon_vma in its rmap_item when adding to stable tree:
all the vmas which might map that page are listed by its anon_vma.

page_referenced_ksm() and try_to_unmap_ksm() then traverse the anon_vma,
first to find the probable vma, that which matches rmap_item's mm; but if
that is not enough to locate all instances, traverse again to try the
others.  This catches those occasions when fork has duplicated a pte of a
ksm page, but ksmd has not yet come around to assign it an rmap_item.

But each rmap_item in the stable tree which refers to an anon_vma needs to
take a reference to it.  Andrea's anon_vma design cleverly avoided a
reference count (an anon_vma was free when its list of vmas was empty),
but KSM now needs to add that.  Is a 32-bit count sufficient?  I believe
so - the anon_vma is only free when both count is 0 and list is empty.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 869aaa3206a2..ebdf582ef185 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -68,7 +68,7 @@ static inline struct anon_vma *anon_vma_alloc(void)
         return kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
 }
 
-static inline void anon_vma_free(struct anon_vma *anon_vma)
+void anon_vma_free(struct anon_vma *anon_vma)
 {
         kmem_cache_free(anon_vma_cachep, anon_vma);
 }
@@ -172,7 +172,7 @@ void anon_vma_unlink(struct vm_area_struct *vma)
         list_del(&vma->anon_vma_node);
 
         /* We must garbage collect the anon_vma if it's empty */
-        empty = list_empty(&anon_vma->head);
+        empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma);
         spin_unlock(&anon_vma->lock);
 
         if (empty)
@@ -184,6 +184,7 @@ static void anon_vma_ctor(void *data)
         struct anon_vma *anon_vma = data;
 
         spin_lock_init(&anon_vma->lock);
+        ksm_refcount_init(anon_vma);
         INIT_LIST_HEAD(&anon_vma->head);
 }