Patch "zram: support BDI_CAP_STABLE_WRITES" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

zram: support BDI_CAP_STABLE_WRITES

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
zram-support-bdi_cap_stable_writes.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Minchan Kim <[email protected]>
Date: Tue, 10 Jan 2017 16:58:21 -0800
Subject: zram: support BDI_CAP_STABLE_WRITES

From: Minchan Kim <[email protected]>

commit b09ab054b69b07077bd3292f67e777861ac796e5 upstream.

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*".

---
 drivers/block/zram/zram_drv.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

@@ -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>

@@ -111,6 +112,14 @@ static inline bool is_partial_io(struct
 	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.
  */

@@ -1094,7 +1103,7 @@ static ssize_t disksize_store(struct 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 +1151,7 @@ static ssize_t reset_store(struct device
 	/* 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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/zram-support-bdi_cap_stable_writes.patch queue-4.9/zram-revalidate-disk-under-init_lock.patch queue-4.9/mm-pmd-dirty-emulation-in-page-fault-handler.patch