1. Add the new helper, compound_head_put_tail(page) which returns
the stable compound_head() and drops the reference to this page
if it is the sub-page of __compound_tail_refcounted(head).
Note: this patch tries to be as simple as possible. But we need
to unify the new helper with __put_page_tail() later, or at least
factor out the common code.
2. Remove the nasty __get_user_pages_fast() code in get_futex_key(),
it can use the new helper to get page_head.
Note: with or without this change basepage_index(page) after
put_page(page) looks confusing at least, this will be addressed
later.
Suggested-by: Andrea Arcangeli <aarca...@redhat.com>
Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
include/linux/mm.h | 2 ++
kernel/futex.c | 37 +------------------------------------
mm/swap.c | 35 +++++++++++++++++++++++++++++++++++
3 files changed, 38 insertions(+), 36 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 13495bd..545df45 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -501,6 +501,8 @@ static inline void __ClearPageBuddy(struct page *page)
void put_page(struct page *page);
void put_pages_list(struct list_head *pages);
+struct page *compound_head_put_tail(struct page *page);
+
void split_page(struct page *page, unsigned int order);
int split_free_page(struct page *page);
diff --git a/kernel/futex.c b/kernel/futex.c
index c3a1a55..1fd7031 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -282,42 +282,7 @@ again:
else
err = 0;
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- page_head = page;
- if (unlikely(PageTail(page))) {
- put_page(page);
- /* serialize against __split_huge_page_splitting() */
- local_irq_disable();
- if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
- page_head = compound_head(page);
- /*
- * page_head is valid pointer but we must pin
- * it before taking the PG_lock and/or
- * PG_compound_lock. The moment we re-enable
- * irqs __split_huge_page_splitting() can
- * return and the head page can be freed from
- * under us. We can't take the PG_lock and/or
- * PG_compound_lock on a page that could be
- * freed from under us.
- */
- if (page != page_head) {
- get_page(page_head);
- put_page(page);
- }
- local_irq_enable();
- } else {
- local_irq_enable();
- goto again;
- }
- }
-#else
- page_head = compound_head(page);
- if (page != page_head) {
- get_page(page_head);
- put_page(page);
- }
-#endif
-
+ page_head = compound_head_put_tail(page);
lock_page(page_head);
/*
diff --git a/mm/swap.c b/mm/swap.c
index 972923d..6742c85 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -215,6 +215,41 @@ void put_page(struct page *page)
EXPORT_SYMBOL(put_page);
/*
+ * Like compound_head() but also drops the additional reference
+ * this page can have. Unlike compound_head() it returns the page
+ * which has a reference, and can't race with split_huge_page().
+ */
+struct page *compound_head_put_tail(struct page *page)
+{
+ struct page *page_head;
+ unsigned long flags;
+
+ if (!PageTail(page))
+ return page;
+
+ page_head = compound_trans_head(page);
+
+ if (!__compound_tail_refcounted(page_head)) {
+ smp_rmb();
+ if (likely(PageTail(page)))
+ return page_head;
+ else
+ return page;
+ }
+
+ if (likely(get_lock_thp_head(page_head, page, &flags))) {
+ if (put_page_testzero(page_head))
+ VM_BUG_ON(1);
+ atomic_dec(&page->_mapcount);
+ compound_unlock_irqrestore(page_head, flags);
+
+ return page_head;
+ }
+
+ return page;
+}
+
+/*
* This function is exported but must not be called by anything other
* than get_page(). It implements the slow path of get_page().
*/
--
1.5.5.1
--
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