[PATCH v2] zram: Add a huge_idle writeback mode

[Brian Geffon <bgeffon@google.com>]

Today it's only possible to write back as a page, idle, or huge.
A user might want to writeback pages which are huge and idle first
as these idle pages do not require decompression and make a good
first pass for writeback.

Idle writeback specifically has the advantage that a refault is
unlikely given that the page has been swapped for some amount of
time without being refaulted.

Huge writeback has the advantage that you're guaranteed to get
the maximum benefit from a single page writeback, that is, you're
reclaiming one full page of memory. Pages which are compressed in
zram being written back result in some benefit which is always
less than a page size because of the fact that it was compressed.

The primary use of this is for minimizing refaults in situations
where the device has to be sensitive to storage endurance. On
ChromeOS we have devices with slow eMMC and repeated writes and
refaults can negatively affect performance and endurance.

Signed-off-by: Brian Geffon <bgeffon@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>
---
 Documentation/admin-guide/blockdev/zram.rst |  5 +++++
 drivers/block/zram/zram_drv.c               | 10 ++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 54fe63745ed8..c73b16930449 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -343,6 +343,11 @@ Admin can request writeback of those idle pages at right timing via::
 
 With the command, zram will writeback idle pages from memory to the storage.
 
+Additionally, if a user choose to writeback only huge and idle pages
+this can be accomplished with::
+
+        echo huge_idle > /sys/block/zramX/writeback
+
 If an admin wants to write a specific page in zram device to the backing device,
 they could write a page index into the interface.
 
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index e9474b02012d..8562a7cce558 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -639,8 +639,8 @@ static int read_from_bdev_async(struct zram *zram, struct bio_vec *bvec,
 #define PAGE_WB_SIG "page_index="
 
 #define PAGE_WRITEBACK 0
-#define HUGE_WRITEBACK 1
-#define IDLE_WRITEBACK 2
+#define HUGE_WRITEBACK (1<<0)
+#define IDLE_WRITEBACK (1<<1)
 
 
 static ssize_t writeback_store(struct device *dev,
@@ -660,6 +660,8 @@ static ssize_t writeback_store(struct device *dev,
 		mode = IDLE_WRITEBACK;
 	else if (sysfs_streq(buf, "huge"))
 		mode = HUGE_WRITEBACK;
+	else if (sysfs_streq(buf, "huge_idle"))
+		mode = IDLE_WRITEBACK | HUGE_WRITEBACK;
 	else {
 		if (strncmp(buf, PAGE_WB_SIG, sizeof(PAGE_WB_SIG) - 1))
 			return -EINVAL;
@@ -721,10 +723,10 @@ static ssize_t writeback_store(struct device *dev,
 				zram_test_flag(zram, index, ZRAM_UNDER_WB))
 			goto next;
 
-		if (mode == IDLE_WRITEBACK &&
+		if (mode & IDLE_WRITEBACK &&
 			  !zram_test_flag(zram, index, ZRAM_IDLE))
 			goto next;
-		if (mode == HUGE_WRITEBACK &&
+		if (mode & HUGE_WRITEBACK &&
 			  !zram_test_flag(zram, index, ZRAM_HUGE))
 			goto next;
 		/*
-- 
2.35.1.894.gb6a874cedc-goog