[PATCH v3 3/3] zram: support BDI_CAP_STABLE_WRITES

[Minchan Kim]

zram has used per-cpu stream feature from v4.7.
It aims for increasing cache hit ratio of scratch buffer for
compressing. Downside of that approach is that zram should ask
memory space for compressed page in per-cpu context which requires
stricted gfp flag which could be failed. If so, it retries to
allocate memory space out of per-cpu context so it could get memory
this time and compress the data again, copies it to the memory space.

In this scenario, zram assumes the data should never be changed
but it is not true without stable page support. So, If the data is
changed under us, zram can make buffer overrun so that zsmalloc
free object chain is broken so system goes crash like below
https://bugzilla.suse.com/show_bug.cgi?id=997574

This patch adds BDI_CAP_STABLE_WRITES to zram for declaring
"I am block device needing *stable write*".

Fixes: da9556a2367c ("zram: user per-cpu compression streams")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 drivers/block/zram/zram_drv.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d93a4b2135c2..9d7f83f9a388 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -30,6 +30,7 @@
 #include <linux/err.h>
 #include <linux/idr.h>
 #include <linux/sysfs.h>
+#include <linux/backing-dev.h>
 
 #include "zram_drv.h"
 
@@ -111,6 +112,13 @@ static inline bool is_partial_io(struct bio_vec *bvec)
 	return bvec->bv_len != PAGE_SIZE;
 }
 
+static void zram_revalidate_disk(struct zram *zram)
+{
+	revalidate_disk(zram->disk);
+	zram->disk->queue->backing_dev_info.capabilities |=
+		BDI_CAP_STABLE_WRITES;
+}
+
 /*
  * Check if request is within bounds and aligned on zram logical blocks.
  */
@@ -1094,7 +1102,7 @@ static ssize_t disksize_store(struct device *dev,
 	zram->comp = comp;
 	zram->disksize = disksize;
 	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
-	revalidate_disk(zram->disk);
+	zram_revalidate_disk(zram);
 	up_write(&zram->init_lock);
 
 	return len;
@@ -1142,7 +1150,7 @@ static ssize_t reset_store(struct device *dev,
 	/* Make sure all the pending I/O are finished */
 	fsync_bdev(bdev);
 	zram_reset_device(zram);
-	revalidate_disk(zram->disk);
+	zram_revalidate_disk(zram);
 	bdput(bdev);
 
 	mutex_lock(&bdev->bd_mutex);
-- 
2.7.4