zram: add a huge_idle writeback mode

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.

Link: https://lkml.kernel.org/r/20220322215821.1196994-1-bgeffon@google.com
Signed-off-by: Brian Geffon <bgeffon@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 54fe63745ed8e57db23336d0a7afd1979d4c7691..c73b16930449e6eb1efe62aa8ab774c113db6343 100644 (file)
--- 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 e9474b02012deb758f55df23859a986072a8dd98..8562a7cce558b86c9d01514696be828a52becdbf 100644 (file)
--- 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;
                /*