Commit 30acbaba authored Jun 27, 2007 by Hugh Dickins Committed by Linus Torvalds Jun 28, 2007

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>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 172d0496

mm/rmap.c

+1 −23

Original line number	Diff line number	Diff line
		@@ -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 */

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