[RFC PATCH 1/2] mm: swap: allow partial swapoff with try_to_unuse()

[Andrea Righi <andrea.righi@canonical.com>]

Allow to run try_to_unuse() passing an arbitrary amount of pages also
when frontswap is not used.

To preserve the default behavior introduce a new function called
try_to_unuse_wait() and add a new 'wait' parameter: if 'wait' is false
return as soon as "pages_to_unuse" pages are unused, if it is true
simply ignore "pages_to_unuse" and wait until all the pages are unused.

In any case the value of 0 in "pages_to_unuse" means "all pages".

This is required by the PM / hibernation opportunistic memory reclaim
feature.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 include/linux/swapfile.h |  7 +++++++
 mm/swapfile.c            | 15 +++++++--------
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
index e06febf62978..ac4d0ccd1f7b 100644
--- a/include/linux/swapfile.h
+++ b/include/linux/swapfile.h
@@ -9,6 +9,13 @@
 extern spinlock_t swap_lock;
 extern struct plist_head swap_active_head;
 extern struct swap_info_struct *swap_info[];
+extern int try_to_unuse_wait(unsigned int type, bool frontswap, bool wait,
+			     unsigned long pages_to_unuse);
+static inline int
+try_to_unuse(unsigned int type, bool frontswap, unsigned long pages_to_unuse)
+{
+	return try_to_unuse_wait(type, frontswap, true, pages_to_unuse);
+}
 extern int try_to_unuse(unsigned int, bool, unsigned long);
 extern unsigned long generic_max_swapfile_size(void);
 extern unsigned long max_swapfile_size(void);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index f8bf926c9c8f..651471ccf133 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2121,10 +2121,13 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
 }
 
 /*
- * If the boolean frontswap is true, only unuse pages_to_unuse pages;
- * pages_to_unuse==0 means all pages; ignored if frontswap is false
+ * Unuse pages_to_unuse pages; pages_to_unuse==0 means all pages.
+ *
+ * If "wait" is false stop as soon as "pages_to_unuse" pages are unused, if
+ * wait is true "pages_to_unuse" will be ignored and wait until all the pages
+ * are unused.
  */
-int try_to_unuse(unsigned int type, bool frontswap,
+int try_to_unuse_wait(unsigned int type, bool frontswap, bool wait,
 		 unsigned long pages_to_unuse)
 {
 	struct mm_struct *prev_mm;
@@ -2138,10 +2141,6 @@ int try_to_unuse(unsigned int type, bool frontswap,
 
 	if (!READ_ONCE(si->inuse_pages))
 		return 0;
-
-	if (!frontswap)
-		pages_to_unuse = 0;
-
 retry:
 	retval = shmem_unuse(type, frontswap, &pages_to_unuse);
 	if (retval)
@@ -2223,7 +2222,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
 	 * been preempted after get_swap_page(), temporarily hiding that swap.
 	 * It's easy and robust (though cpu-intensive) just to keep retrying.
 	 */
-	if (READ_ONCE(si->inuse_pages)) {
+	if (wait && READ_ONCE(si->inuse_pages)) {
 		if (!signal_pending(current))
 			goto retry;
 		retval = -EINTR;
-- 
2.25.1