Re: [PATCHv4 14/39] thp, mm: rewrite delete_from_page_cache() to support huge pages

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Kirill A. Shutemov wrote:
> Dave Hansen wrote:
> > Which reminds me...  Why do we handle their reference counts differently? :)
> > 
> > It seems like we could easily put a for loop in delete_from_page_cache()
> > that will release their reference counts along with the head page.
> > Wouldn't that make the code less special-cased for tail pages?
> 
> delete_from_page_cache() is not the only user of
> __delete_from_page_cache()...
> 
> It seems I did it wrong in add_to_page_cache_locked(). We shouldn't take
> references on tail pages there, only one on head. On split it will be
> distributed properly.

This way:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b267859..c2c0df2 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1556,6 +1556,7 @@ static void __split_huge_page_refcount(struct page *page,
 	struct zone *zone = page_zone(page);
 	struct lruvec *lruvec;
 	int tail_count = 0;
+	int init_tail_refcount;
 
 	/* prevent PageLRU to go away from under us, and freeze lru stats */
 	spin_lock_irq(&zone->lru_lock);
@@ -1565,6 +1566,13 @@ static void __split_huge_page_refcount(struct page *page,
 	/* complete memcg works before add pages to LRU */
 	mem_cgroup_split_huge_fixup(page);
 
+	/*
+	 * When we add a huge page to page cache we take only reference to head
+	 * page, but on split we need to take addition reference to all tail
+	 * pages since they are still in page cache after splitting.
+	 */
+	init_tail_refcount = PageAnon(page) ? 0 : 1;
+
 	for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
 		struct page *page_tail = page + i;
 
@@ -1587,8 +1595,9 @@ static void __split_huge_page_refcount(struct page *page,
 		 * atomic_set() here would be safe on all archs (and
 		 * not only on x86), it's safer to use atomic_add().
 		 */
-		atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1,
-			   &page_tail->_count);
+		atomic_add(init_tail_refcount + page_mapcount(page) +
+				page_mapcount(page_tail) + 1,
+				&page_tail->_count);
 
 		/* after clearing PageTail the gup refcount can be released */
 		smp_mb();
-- 
 Kirill A. Shutemov

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