[PATCH v2] zram: support a page writeback

[Minchan Kim]

There is a demand to writeback specific pages on process to backing store
instead of all idles pages in the system due to storage wear out concern
and launching latency of apps which are most of time idle but critical
for resume latency.

This patch extends the writeback knob to support a specific page
writeback.

Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---

* from v1
  * use "page_index=" signature for future extension - Sergey

 Documentation/admin-guide/blockdev/zram.rst |  5 +++++
 drivers/block/zram/zram_drv.c               | 21 +++++++++++++++++----
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index a6fd1f9b5faf..8f3cfa42e443 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -334,6 +334,11 @@ IOW, unless there is access request, those pages are still idle pages.
 
 With the command, zram writeback idle pages from memory to the storage.
 
+If admin want to write a specific page in zram device to backing device,
+they could write a page index into the interface.
+
+	echo "page_index=1251" > /sys/block/zramX/writeback
+
 If there are lots of write IO with flash device, potentially, it has
 flash wearout problem so that admin needs to design write limitation
 to guarantee storage health for entire product life.
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 9100ac36670a..1e542008e798 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -618,15 +618,19 @@ static int read_from_bdev_async(struct zram *zram, struct bio_vec *bvec,
 	return 1;
 }
 
+#define PAGE_WB_SIG "page_index="
+
+#define PAGE_WRITEBACK 0
 #define HUGE_WRITEBACK 1
 #define IDLE_WRITEBACK 2
 
+
 static ssize_t writeback_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t len)
 {
 	struct zram *zram = dev_to_zram(dev);
 	unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
-	unsigned long index;
+	unsigned long index = 0;
 	struct bio bio;
 	struct bio_vec bio_vec;
 	struct page *page;
@@ -638,8 +642,17 @@ static ssize_t writeback_store(struct device *dev,
 		mode = IDLE_WRITEBACK;
 	else if (sysfs_streq(buf, "huge"))
 		mode = HUGE_WRITEBACK;
-	else
-		return -EINVAL;
+	else {
+		if (strncmp(buf, PAGE_WB_SIG, sizeof(PAGE_WB_SIG) - 1))
+			return -EINVAL;
+
+		ret = kstrtol(buf + sizeof(PAGE_WB_SIG) - 1, 10, &index);
+		if (ret || index >= nr_pages)
+			return -EINVAL;
+
+		nr_pages = 1;
+		mode = PAGE_WRITEBACK;
+	}
 
 	down_read(&zram->init_lock);
 	if (!init_done(zram)) {
@@ -658,7 +671,7 @@ static ssize_t writeback_store(struct device *dev,
 		goto release_init_lock;
 	}
 
-	for (index = 0; index < nr_pages; index++) {
+	while (nr_pages--) {
 		struct bio_vec bvec;
 
 		bvec.bv_page = page;
-- 
2.29.0.rc1.297.gfa9743e501-goog