[RFC PATCH] zram: update ac_time of written back slots

[Sergey Senozhatsky <senozhatsky@chromium.org>]

This makes it possible to tell how long each particular
ZRAM_WB slot stays on the disk and, basically, to monitor
writeback efficiency.

Note that the patch factors out a small update_slot_ac_time()
helper, which we call on writeback completion, instead of
calling mark_slot_accessed().  The reasons being is that
mark_slot_accessed() clears ZRAM_PP_SLOT flag ahead of time,
while we want to keep that flag set as long as the slot is on
the post-processing list (unless the slot is freed concurrently).

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Brian Geffon <bgeffon@google.com>
---
 drivers/block/zram/zram_drv.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 5ecc4ba40e9d..dcea703a6766 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -185,15 +185,20 @@ static inline u32 get_slot_comp_priority(struct zram *zram, u32 index)
 	return prio & ZRAM_COMP_PRIORITY_MASK;
 }
 
-static void mark_slot_accessed(struct zram *zram, u32 index)
+static void update_slot_ac_time(struct zram *zram, u32 index)
 {
-	clear_slot_flag(zram, index, ZRAM_IDLE);
-	clear_slot_flag(zram, index, ZRAM_PP_SLOT);
 #ifdef CONFIG_ZRAM_TRACK_ENTRY_ACTIME
 	zram->table[index].attr.ac_time = (u32)ktime_get_boottime_seconds();
 #endif
 }
 
+static void mark_slot_accessed(struct zram *zram, u32 index)
+{
+	clear_slot_flag(zram, index, ZRAM_IDLE);
+	clear_slot_flag(zram, index, ZRAM_PP_SLOT);
+	update_slot_ac_time(zram, index);
+}
+
 static inline void update_used_max(struct zram *zram, const unsigned long pages)
 {
 	unsigned long cur_max = atomic_long_read(&zram->stats.max_used_pages);
@@ -952,6 +957,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 	zs_free(zram->mem_pool, get_slot_handle(zram, index));
 	set_slot_handle(zram, index, req->blk_idx);
 	set_slot_flag(zram, index, ZRAM_WB);
+	update_slot_ac_time(zram, index);
 
 out:
 	slot_unlock(zram, index);
-- 
2.53.0.1018.g2bb0e51243-goog