ksm: 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 <[email protected]>
Signed-off-by: Rik van Riel <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: Avi Kivity <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 43bdab769fc358091f90315d2613c40ada107da0..74d691ee9121c5bb3aa8336d7eeffcc03d88cc46 100644 (file)
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -78,7 +78,7 @@ static inline struct page *ksm_might_need_to_copy(struct page *page,
        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;