[PATCH 6.1/7] mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split

[Hugh Dickins]

From: Yang Shi <shy828301@xxxxxxxxx>

When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however
it may miss the failure when head page is unmapped but other subpage is
mapped.  Then the second DEBUG_VM BUG() that check total mapcount would
catch it.  This may incur some confusion.  And this is not a fatal issue,
so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE().

[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@xxxxxxxxxxxx/

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
v5: Rediffed by Hugh to fit after 6/7 in his mm/thp series; Cc stable.
v4: Updated the subject and commit log per Hugh.
    Reordered the patches per Hugh.
v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
    since there is no fundamental change against v2.
v2: Removed dead code and updated the comment of try_to_unmap() per Zi
    Yan.

 mm/huge_memory.c | 26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2350,19 +2350,19 @@ static void unmap_page(struct page *page
 {
 	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
 		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
-	bool unmap_success;
 
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 
 	if (PageAnon(page))
 		ttu_flags |= TTU_SPLIT_FREEZE;
 
-	/* Make sure that the BUGs will not bite */
+	/* If warning below, prevent a race in the mapped accounting */
 	if (IS_ENABLED(CONFIG_DEBUG_VM))
 		ttu_flags |= TTU_SYNC;
 
-	unmap_success = try_to_unmap(page, ttu_flags);
-	VM_BUG_ON_PAGE(!unmap_success, page);
+	try_to_unmap(page, ttu_flags);
+
+	VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
 }
 
 static void remap_page(struct page *page, unsigned int nr)
@@ -2673,7 +2673,7 @@ int split_huge_page_to_list(struct page
 	struct deferred_split *ds_queue = get_deferred_split_queue(head);
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
-	int count, mapcount, extra_pins, ret;
+	int extra_pins, ret;
 	pgoff_t end;
 
 	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
@@ -2732,7 +2732,6 @@ int split_huge_page_to_list(struct page
 	}
 
 	unmap_page(head);
-	VM_BUG_ON_PAGE(compound_mapcount(head), head);
 
 	/* block interrupt reentry in xa_lock and spinlock */
 	local_irq_disable();
@@ -2750,9 +2749,7 @@ int split_huge_page_to_list(struct page
 
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	spin_lock(&ds_queue->split_queue_lock);
-	count = page_count(head);
-	mapcount = total_mapcount(head);
-	if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
+	if (page_ref_freeze(head, 1 + extra_pins)) {
 		if (!list_empty(page_deferred_list(head))) {
 			ds_queue->split_queue_len--;
 			list_del(page_deferred_list(head));
@@ -2772,16 +2769,9 @@ int split_huge_page_to_list(struct page
 		__split_huge_page(page, list, end);
 		ret = 0;
 	} else {
-		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
-			pr_alert("total_mapcount: %u, page_count(): %u\n",
-					mapcount, count);
-			if (PageTail(page))
-				dump_page(head, NULL);
-			dump_page(page, "total_mapcount(head) > 0");
-			BUG();
-		}
 		spin_unlock(&ds_queue->split_queue_lock);
-fail:		if (mapping)
+fail:
+		if (mapping)
 			xa_unlock(&mapping->i_pages);
 		local_irq_enable();
 		remap_page(head, thp_nr_pages(head));