[PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

[Kirill A. Shutemov]

We can collapse PTE-mapped compound pages. We only need to avoid
handling them more than once: lock/unlock page only once if it's present
in the PMD range multiple times as it handled on compound level. The
same goes for LRU isolation and putpack.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b47edfe57f7b..c8c2c463095c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)
 
 static void release_pte_page(struct page *page)
 {
+	/*
+	 * We need to unlock and put compound page on LRU only once.
+	 * The rest of the pages have to be locked and not on LRU here.
+	 */
+	VM_BUG_ON_PAGE(!PageCompound(page) &&
+			(!PageLocked(page) && PageLRU(page)), page);
+
+	if (!PageLocked(page))
+		return;
+
+	page = compound_head(page);
 	dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
 	unlock_page(page);
 	putback_lru_page(page);
@@ -537,6 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	pte_t *_pte;
 	int none_or_zero = 0, result = 0, referenced = 0;
 	bool writable = false;
+	LIST_HEAD(compound_pagelist);
 
 	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
 	     _pte++, address += PAGE_SIZE) {
@@ -561,13 +573,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			goto out;
 		}
 
-		/* TODO: teach khugepaged to collapse THP mapped with pte */
+		VM_BUG_ON_PAGE(!PageAnon(page), page);
+
 		if (PageCompound(page)) {
-			result = SCAN_PAGE_COMPOUND;
-			goto out;
-		}
+			struct page *p;
+			page = compound_head(page);
 
-		VM_BUG_ON_PAGE(!PageAnon(page), page);
+			/*
+			 * Check if we have dealt with the compount page
+			 * already
+			 */
+			list_for_each_entry(p, &compound_pagelist, lru) {
+				if (page ==  p)
+					break;
+			}
+			if (page ==  p)
+				continue;
+		}
 
 		/*
 		 * We can do it before isolate_lru_page because the
@@ -640,6 +662,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		    page_is_young(page) || PageReferenced(page) ||
 		    mmu_notifier_test_young(vma->vm_mm, address))
 			referenced++;
+
+		if (PageCompound(page))
+			list_add_tail(&page->lru, &compound_pagelist);
 	}
 	if (likely(writable)) {
 		if (likely(referenced)) {
@@ -1185,11 +1210,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
-		/* TODO: teach khugepaged to collapse THP mapped with pte */
-		if (PageCompound(page)) {
-			result = SCAN_PAGE_COMPOUND;
-			goto out_unmap;
-		}
+		page = compound_head(page);
 
 		/*
 		 * Record which node the original page is from and save this
-- 
2.26.0