[PATCH 6.2/7] mm: rmap: make try_to_unmap() void function

[PATCH 6.2/7] mm: rmap: make try_to_unmap() void function

[Hugh Dickins]

From: Yang Shi <shy828301@gmail.com>

Currently try_to_unmap() return bool value by checking page_mapcount(),
however this may return false positive since page_mapcount() doesn't
check all subpages of compound page.  The total_mapcount() could be used
instead, but its cost is higher since it traverses all subpages.

Actually the most callers of try_to_unmap() don't care about the
return value at all.  So just need check if page is still mapped by
page_mapped() when necessary.  And page_mapped() does bail out early
when it finds mapped subpage.

Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
v5: Rediffed by Hugh to fit before 7/7 in mm/thp series; akpm fixed grammar.
v4: Updated the comment of try_to_unmap() per Minchan.
    Minor fix and patch reorder per Hugh.
    Collected ack tag from Hugh.
 
 include/linux/rmap.h |  2 +-
 mm/memory-failure.c  | 15 +++++++--------
 mm/rmap.c            | 15 ++++-----------
 mm/vmscan.c          |  3 ++-
 4 files changed, 14 insertions(+), 21 deletions(-)

--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -195,7 +195,7 @@ static inline void page_dup_rmap(struct
 int page_referenced(struct page *, int is_locked,
 			struct mem_cgroup *memcg, unsigned long *vm_flags);
 
-bool try_to_unmap(struct page *, enum ttu_flags flags);
+void try_to_unmap(struct page *, enum ttu_flags flags);
 
 /* Avoid racy checks */
 #define PVMW_SYNC		(1 << 0)
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1063,7 +1063,7 @@ static bool hwpoison_user_mappings(struc
 	enum ttu_flags ttu = TTU_IGNORE_MLOCK;
 	struct address_space *mapping;
 	LIST_HEAD(tokill);
-	bool unmap_success = true;
+	bool unmap_success;
 	int kill = 1, forcekill;
 	struct page *hpage = *hpagep;
 	bool mlocked = PageMlocked(hpage);
@@ -1126,7 +1126,7 @@ static bool hwpoison_user_mappings(struc
 		collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
 
 	if (!PageHuge(hpage)) {
-		unmap_success = try_to_unmap(hpage, ttu);
+		try_to_unmap(hpage, ttu);
 	} else {
 		if (!PageAnon(hpage)) {
 			/*
@@ -1138,17 +1138,16 @@ static bool hwpoison_user_mappings(struc
 			 */
 			mapping = hugetlb_page_mapping_lock_write(hpage);
 			if (mapping) {
-				unmap_success = try_to_unmap(hpage,
-						     ttu|TTU_RMAP_LOCKED);
+				try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
 				i_mmap_unlock_write(mapping);
-			} else {
+			} else
 				pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
-				unmap_success = false;
-			}
 		} else {
-			unmap_success = try_to_unmap(hpage, ttu);
+			try_to_unmap(hpage, ttu);
 		}
 	}
+
+	unmap_success = !page_mapped(hpage);
 	if (!unmap_success)
 		pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
 		       pfn, page_mapcount(hpage));
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1405,7 +1405,7 @@ static bool try_to_unmap_one(struct page
 	/*
 	 * When racing against e.g. zap_pte_range() on another cpu,
 	 * in between its ptep_get_and_clear_full() and page_remove_rmap(),
-	 * try_to_unmap() may return false when it is about to become true,
+	 * try_to_unmap() may return before page_mapped() has become false,
 	 * if page table locking is skipped: use TTU_SYNC to wait for that.
 	 */
 	if (flags & TTU_SYNC)
@@ -1756,9 +1756,10 @@ static int page_not_mapped(struct page *
  * Tries to remove all the page table entries which are mapping this
  * page, used in the pageout path.  Caller must hold the page lock.
  *
- * If unmap is successful, return true. Otherwise, false.
+ * It is the caller's responsibility to check if the page is still
+ * mapped when needed (use TTU_SYNC to prevent accounting races).
  */
-bool try_to_unmap(struct page *page, enum ttu_flags flags)
+void try_to_unmap(struct page *page, enum ttu_flags flags)
 {
 	struct rmap_walk_control rwc = {
 		.rmap_one = try_to_unmap_one,
@@ -1783,14 +1784,6 @@ bool try_to_unmap(struct page *page, enu
 		rmap_walk_locked(page, &rwc);
 	else
 		rmap_walk(page, &rwc);
-
-	/*
-	 * When racing against e.g. zap_pte_range() on another cpu,
-	 * in between its ptep_get_and_clear_full() and page_remove_rmap(),
-	 * try_to_unmap() may return false when it is about to become true,
-	 * if page table locking is skipped: use TTU_SYNC to wait for that.
-	 */
-	return !page_mapcount(page);
 }
 
 /**
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1499,7 +1499,8 @@ static unsigned int shrink_page_list(str
 			if (unlikely(PageTransHuge(page)))
 				flags |= TTU_SPLIT_HUGE_PMD;
 
-			if (!try_to_unmap(page, flags)) {
+			try_to_unmap(page, flags);
+			if (page_mapped(page)) {
 				stat->nr_unmap_fail += nr_pages;
 				if (!was_swapbacked && PageSwapBacked(page))
 					stat->nr_lazyfree_fail += nr_pages;