[PATCH 16/28] CacheFiles: Permit the page lock state to be monitored [try #2]

[David Howells <dhowells@redhat.com>]

Add a function to install a monitor on the page lock waitqueue for a particular
page, thus allowing the page being unlocked to be detected.

This is used by CacheFiles to detect read completion on a page in the backing
filesystem so that it can then copy the data to the waiting netfs page.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 include/linux/pagemap.h |    5 +++++
 mm/filemap.c            |   18 ++++++++++++++++++
 2 files changed, 23 insertions(+), 0 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 6a1b317..21c35e2 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -223,6 +223,11 @@ static inline void wait_on_page_fscache_write(struct page *page)
 extern void end_page_fscache_write(struct page *page);
 
 /*
+ * Add an arbitrary waiter to a page's wait queue
+ */
+extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
+
+/*
  * Fault a userspace page into pagetables.  Return non-zero on a fault.
  *
  * This assumes that two userspace pages are always sufficient.  That's
diff --git a/mm/filemap.c b/mm/filemap.c
index bea1ba6..6872d1b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -521,6 +521,24 @@ void fastcall wait_on_page_bit(struct page *page, int bit_nr)
 EXPORT_SYMBOL(wait_on_page_bit);
 
 /**
+ * add_page_wait_queue - Add an arbitrary waiter to a page's wait queue
+ * @page - Page defining the wait queue of interest
+ * @waiter - Waiter to add to the queue
+ *
+ * Add an arbitrary @waiter to the wait queue for the nominated @page.
+ */
+void add_page_wait_queue(struct page *page, wait_queue_t *waiter)
+{
+	wait_queue_head_t *q = page_waitqueue(page);
+	unsigned long flags;
+
+	spin_lock_irqsave(&q->lock, flags);
+	__add_wait_queue(q, waiter);
+	spin_unlock_irqrestore(&q->lock, flags);
+}
+EXPORT_SYMBOL_GPL(add_page_wait_queue);
+
+/**
  * unlock_page - unlock a locked page
  * @page: the page
  *