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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