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Subject[2.6.20.16 review 23/28] mm: kill validate_anon_vma to avoid mapcount BUG
validate_anon_vma gave a useful check on the integrity of the anon_vma list
when Andrea was developing obj rmap; but it was not enabled in SLES9
itself, nor in mainline, until Nick changed commented-out RMAP_DEBUG to
configurable CONFIG_DEBUG_VM in 2.6.17. Now Petr Vandrovec reports that
its BUG_ON(mapcount > 100000) can easily crash a CONFIG_DEBUG_VM=y system.
That limit was just an arbitrary number to protect against an infinite
loop. We could raise it to something enormous (depending on sizeof struct
vma and size of memory?); but I rather think validate_anon_vma has outlived
its usefulness, and is better just removed - which gives a magnificent
performance boost to anything like Petr's test program ;)

Of course, a very long anon_vma list is bad news for preemption latency,
and I believe there has been one recent report of such: let's not forget
that, but validate_anon_vma only makes it worse not better.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Petr Vandrovec <petr@vmware.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: Andrea Arcangeli <andrea@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
mm/rmap.c | 24 +-----------------------
1 files changed, 1 insertions(+), 23 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 7ce69c1..c30781c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -53,24 +53,6 @@

struct kmem_cache *anon_vma_cachep;

-static inline void validate_anon_vma(struct vm_area_struct *find_vma)
-{
-#ifdef CONFIG_DEBUG_VM
- struct anon_vma *anon_vma = find_vma->anon_vma;
- struct vm_area_struct *vma;
- unsigned int mapcount = 0;
- int found = 0;
-
- list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
- mapcount++;
- BUG_ON(mapcount > 100000);
- if (vma == find_vma)
- found = 1;
- }
- BUG_ON(!found);
-#endif
-}
-
/* This must be called under the mmap_sem. */
int anon_vma_prepare(struct vm_area_struct *vma)
{
@@ -121,10 +103,8 @@ void __anon_vma_link(struct vm_area_struct *vma)
{
struct anon_vma *anon_vma = vma->anon_vma;

- if (anon_vma) {
+ if (anon_vma)
list_add_tail(&vma->anon_vma_node, &anon_vma->head);
- validate_anon_vma(vma);
- }
}

void anon_vma_link(struct vm_area_struct *vma)
@@ -134,7 +114,6 @@ void anon_vma_link(struct vm_area_struct *vma)
if (anon_vma) {
spin_lock(&anon_vma->lock);
list_add_tail(&vma->anon_vma_node, &anon_vma->head);
- validate_anon_vma(vma);
spin_unlock(&anon_vma->lock);
}
}
@@ -148,7 +127,6 @@ void anon_vma_unlink(struct vm_area_struct *vma)
return;

spin_lock(&anon_vma->lock);
- validate_anon_vma(vma);
list_del(&vma->anon_vma_node);

/* We must garbage collect the anon_vma if it's empty */
--
1.5.2.4
--
-
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