[RFC] shared page table for hugetlbpage memory causing leak.

[Larry Woodman <lwoodman@redhat.com>]

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I think the shared page table code for hugetlb memory on x86 and x86_64
is causing a leak.  When a user of hugepages exits using this code the 
system
leaks some of the hugepages.

-------------------------------------------------------
Part of /proc/meminfo just before database startup:
HugePages_Total:  5500
HugePages_Free:   5500
HugePages_Rsvd:      0
Hugepagesize:     2048 kB

Just before shutdown:
HugePages_Total:  5500
HugePages_Free:   4475
HugePages_Rsvd:      0
Hugepagesize:     2048 kB

After shutdown:
HugePages_Total:  5500 
HugePages_Free:   4988 
HugePages_Rsvd:      0 
Hugepagesize:     2048 kB
----------------------------------------------------------

I think the problem occurs durring a fork, in copy_hugetlb_page_range(). 
It locates the dst_pte using huge_pte_alloc().  Since huge_pte_alloc() 
calls huge_pmd_share() it will share the pmd page if can yet the main 
loop in copy_hugetlb_page_range() does a get_page() on every hugepage.  
This is a violation of the shared hugepmd pagetable protocol and creates 
additional referenced to the hugepages.  

I think we can skip the entire replication of the ptes when the hugepage
pagetables are shared.  This patch skips copying the ptes and the get_page()
calls if the hugetlbpage pagetable is shared.





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--- linux-2.6.23/mm/hugetlb.c.orig	2008-01-16 12:05:41.496448000 -0500
+++ linux-2.6.23/mm/hugetlb.c	2008-01-16 12:09:57.184746000 -0500
@@ -377,18 +377,22 @@ int copy_hugetlb_page_range(struct mm_st
 		dst_pte = huge_pte_alloc(dst, addr);
 		if (!dst_pte)
 			goto nomem;
-		spin_lock(&dst->page_table_lock);
-		spin_lock(&src->page_table_lock);
-		if (!pte_none(*src_pte)) {
-			if (cow)
-				ptep_set_wrprotect(src, addr, src_pte);
-			entry = *src_pte;
-			ptepage = pte_page(entry);
-			get_page(ptepage);
-			set_huge_pte_at(dst, addr, dst_pte, entry);
+
+		/* if hugetlbpage pagetables are shared dont take additional references */
+		if(!(is_vm_hugtlb_page(vma) && dst_pte == src_pte)) {
+			spin_lock(&dst->page_table_lock);
+			spin_lock(&src->page_table_lock);
+			if (!pte_none(*src_pte)) {
+				if (cow)
+					ptep_set_wrprotect(src, addr, src_pte);
+				entry = *src_pte;
+				ptepage = pte_page(entry);
+				get_page(ptepage);
+				set_huge_pte_at(dst, addr, dst_pte, entry);
+			}
+			spin_unlock(&src->page_table_lock);
+			spin_unlock(&dst->page_table_lock);
 		}
-		spin_unlock(&src->page_table_lock);
-		spin_unlock(&dst->page_table_lock);
 	}
 	return 0;