bdi: introduce BDI_CAP_SYNCHRONOUS_IO

As discussed at

  https://lkml.kernel.org/r/<20170728165604.10455-1-ross.zwisler@linux.intel.com>

someday we will remove rw_page().  If so, we need something to detect
such super-fast storage on which synchronous IO operations like the
current rw_page are always a win.

Introduces BDI_CAP_SYNCHRONOUS_IO to indicate such devices.  With it, we
could use various optimization techniques.

Link: http://lkml.kernel.org/r/1505886205-9671-3-git-send-email-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 8 insertions, 1 deletions

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index c1cf877..588360d 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -20,6 +20,7 @@
 #include <linux/radix-tree.h>
 #include <linux/fs.h>
 #include <linux/slab.h>
+#include <linux/backing-dev.h>
 #ifdef CONFIG_BLK_DEV_RAM_DAX
 #include <linux/pfn_t.h>
 #include <linux/dax.h>
@@ -448,6 +449,7 @@ static struct brd_device *brd_alloc(int i)
 	disk->flags		= GENHD_FL_EXT_DEVT;
 	sprintf(disk->disk_name, "ram%d", i);
 	set_capacity(disk, rd_size * 2);
+	disk->queue->backing_dev_info->capabilities |= BDI_CAP_SYNCHRONOUS_IO;
 
 #ifdef CONFIG_BLK_DEV_RAM_DAX
 	queue_flag_set_unlocked(QUEUE_FLAG_DAX, brd->brd_queue);
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d95bb8c..8856422 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1558,7 +1558,7 @@ static int zram_add(void)
 		blk_queue_max_write_zeroes_sectors(zram->disk->queue, UINT_MAX);
 
 	zram->disk->queue->backing_dev_info->capabilities |=
-					BDI_CAP_STABLE_WRITES;
+			(BDI_CAP_STABLE_WRITES | BDI_CAP_SYNCHRONOUS_IO);
 	add_disk(zram->disk);
 
 	ret = sysfs_create_group(&disk_to_dev(zram->disk)->kobj,
diff --git a/drivers/nvdimm/btt.c b/drivers/nvdimm/btt.c
index d5612bd..e949e33 100644
--- a/drivers/nvdimm/btt.c
+++ b/drivers/nvdimm/btt.c
@@ -23,6 +23,7 @@
 #include <linux/ndctl.h>
 #include <linux/fs.h>
 #include <linux/nd.h>
+#include <linux/backing-dev.h>
 #include "btt.h"
 #include "nd.h"
 
@@ -1402,6 +1403,8 @@ static int btt_blk_init(struct btt *btt)
 	btt->btt_disk->private_data = btt;
 	btt->btt_disk->queue = btt->btt_queue;
 	btt->btt_disk->flags = GENHD_FL_EXT_DEVT;
+	btt->btt_disk->queue->backing_dev_info->capabilities |=
+			BDI_CAP_SYNCHRONOUS_IO;
 
 	blk_queue_make_request(btt->btt_queue, btt_make_request);
 	blk_queue_logical_block_size(btt->btt_queue, btt->sector_size);
diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index 39dfd7a..7fbc5c5 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -31,6 +31,7 @@
 #include <linux/uio.h>
 #include <linux/dax.h>
 #include <linux/nd.h>
+#include <linux/backing-dev.h>
 #include "pmem.h"
 #include "pfn.h"
 #include "nd.h"
@@ -394,6 +395,7 @@ static int pmem_attach_disk(struct device *dev,
 	disk->fops		= &pmem_fops;
 	disk->queue		= q;
 	disk->flags		= GENHD_FL_EXT_DEVT;
+	disk->queue->backing_dev_info->capabilities |= BDI_CAP_SYNCHRONOUS_IO;
 	nvdimm_namespace_disk_name(ndns, disk->disk_name);
 	set_capacity(disk, (pmem->size - pmem->pfn_pad - pmem->data_offset)
 			/ 512);