commit d32f35cbc5a87af5e1100dd55074a79fc2cb5307
author Peter Lieven <pl@kamp.de> Thu Oct 24 12:06:52 2013 +0200
committer Kevin Wolf <kwolf@redhat.com> Thu Nov 28 10:30:51 2013 +0100
tree 55fa60fcbf692789af57904cb5bc84be7b649df7
parent aa7bfbfff792538a9eeefe879fc4c629aa0b4203

block: introduce BDRV_REQ_MAY_UNMAP request flag

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block-migration.c

diff --git a/block-migration.c b/block-migration.c
index 713a8e3..fc4ef93 100644
--- a/block-migration.c
+++ b/block-migration.c
@@ -780,7 +780,8 @@
             }
 
             if (flags & BLK_MIG_FLAG_ZERO_BLOCK) {
-                ret = bdrv_write_zeroes(bs, addr, nr_sectors, 0);
+                ret = bdrv_write_zeroes(bs, addr, nr_sectors,
+                                        BDRV_REQ_MAY_UNMAP);
             } else {
                 buf = g_malloc(BLOCK_SIZE);
                 qemu_get_buffer(f, buf, BLOCK_SIZE);

block.c

diff --git a/block.c b/block.c
index e14ba48..d34e974 100644
--- a/block.c
+++ b/block.c
@@ -2818,6 +2818,10 @@
 {
     trace_bdrv_co_write_zeroes(bs, sector_num, nb_sectors);
 
+    if (!(bs->open_flags & BDRV_O_UNMAP)) {
+        flags &= ~BDRV_REQ_MAY_UNMAP;
+    }
+
     return bdrv_co_do_writev(bs, sector_num, nb_sectors, NULL,
                              BDRV_REQ_ZERO_WRITE | flags);
 }

block/backup.c

diff --git a/block/backup.c b/block/backup.c
index 830a179..0198514 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -139,7 +139,7 @@
         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
             ret = bdrv_co_write_zeroes(job->target,
                                        start * BACKUP_SECTORS_PER_CLUSTER,
-                                       n, 0);
+                                       n, BDRV_REQ_MAY_UNMAP);
         } else {
             ret = bdrv_co_writev(job->target,
                                  start * BACKUP_SECTORS_PER_CLUSTER, n,

include/block/block.h

diff --git a/include/block/block.h b/include/block/block.h
index 8ba9f0c..1f30a56 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -65,6 +65,13 @@
 typedef enum {
     BDRV_REQ_COPY_ON_READ = 0x1,
     BDRV_REQ_ZERO_WRITE   = 0x2,
+    /* The BDRV_REQ_MAY_UNMAP flag is used to indicate that the block driver
+     * is allowed to optimize a write zeroes request by unmapping (discarding)
+     * blocks if it is guaranteed that the result will read back as
+     * zeroes. The flag is only passed to the driver if the block device is
+     * opened with BDRV_O_UNMAP.
+     */
+    BDRV_REQ_MAY_UNMAP    = 0x4,
 } BdrvRequestFlags;
 
 #define BDRV_O_RDWR        0x0002