On Sun, Feb 19, 2012 at 01:21:02PM -0800, Hugh Dickins wrote:

On Wed, 8 Feb 2012, Naoya Horiguchi wrote:

quoted

Currently when we check if we can handle thp as it is or we need to
split it into regular sized pages, we hold page table lock prior to
check whether a given pmd is mapping thp or not. Because of this,
when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead.
To remove it, this patch introduces a optimized check function and
replace several similar logics with it.

Signed-off-by: Naoya Horiguchi <redacted>
Cc: David Rientjes <rientjes@google.com>

Changes since v4:
  - Rethink returned value of __pmd_trans_huge_lock()

[snip]

quoted

--- 3.3-rc2.orig/mm/mremap.c
+++ 3.3-rc2/mm/mremap.c

@@ -155,8 +155,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 			if (err > 0) {
 				need_flush = true;
 				continue;
-			} else if (!err) {
-				split_huge_page_pmd(vma->vm_mm, old_pmd);
 			}
 			VM_BUG_ON(pmd_trans_huge(*old_pmd));
 		}

Thanks for reporting, 

Is that what you intended to do there?

No. This is a bug.

I just hit that VM_BUG_ON on rc3-next-20120217.

I found that when extend != HPAGE_PMD_SIZE, thp is not split so
it hits the VM_BUG_ON.
The following patch cancels the change in returned value in v4->v5
and I confirmed this fixes the problem in my simple test.
Andrew, could you add it on top of this optimization patch?

Naoya
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