[PATCH -mm 3/6] ksm: fix ksm swapin time optimization

[Rik van Riel <riel@redhat.com>]

From: Andrea Arcangeli <aarcange@redhat.com>
Subject: fix ksm swapin time optimization

The new anon-vma code, was suboptimal and it lead to erratic invocation of
ksm_does_need_to_copy. That leads to host hangs or guest vnc lockup, or weird
behavior.  It's unclear why ksm_does_need_to_copy is unstable but the point is
that when KSM is not in use, ksm_does_need_to_copy must never run or we bounce
pages for no good reason. I suspect the same hangs will happen with KVM swaps.
But this at least fixes the regression in the new-anon-vma code and it only let
KSM bugs triggers when KSM is in use.

The code in do_swap_page likely doesn't cope well with a not-swapcache,
especially the memcg code.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
---

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -78,7 +78,7 @@ static inline struct page *ksm_might_nee
 	struct anon_vma *anon_vma = page_anon_vma(page);
 
 	if (!anon_vma ||
-	    (anon_vma == vma->anon_vma &&
+	    (anon_vma->root == vma->anon_vma->root &&
 	     page->index == linear_page_index(vma, address)))
 		return page;
 

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