zram: support BDI_CAP_STABLE_WRITES

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")
Link: http://lkml.kernel.org/r/1482366980-3782-4-git-send-email-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Hyeoncheol Lee <cheol.lee@lge.com>
Cc: <yjay.kim@lge.com>
Cc: Sangseok Lee <sangseok.lee@lge.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: <stable@vger.kernel.org> [4.7+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -25,6 +25,7 @@
 #include <linux/genhd.h>
 #include <linux/highmem.h>
 #include <linux/slab.h>
+#include <linux/backing-dev.h>
 #include <linux/string.h>
 #include <linux/vmalloc.h>
 #include <linux/err.h>
@@ -112,6 +113,14 @@ 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);
+	/* revalidate_disk reset the BDI_CAP_STABLE_WRITES so set again */
+	zram->disk->queue->backing_dev_info.capabilities |=
+		BDI_CAP_STABLE_WRITES;
+}
+
 /*
  * Check if request is within bounds and aligned on zram logical blocks.
  */
@@ -1095,7 +1104,7 @@ 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;
@@ -1143,7 +1152,7 @@ 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);