[RFC] block/mirror: fix file-system went to read-only after block-mirror

[Jinhua Cao]

1) Configure the VM disk as prdm.
...
<disk type='block' device='lun'>
  <driver name='qemu' type='raw' cache='none'/>
  <source dev='/dev/disk/by-id/scsi-368886030000000ca50c1cd1563996917' 
index='1'/>
  <backingStore/>
  <target dev='sdb' bus='scsi'/>
  <alias name='scsi0-0-0-1'/>
  <address type='drive' controller='0' bus='0' target='0' unit='1'/>
</disk>
...
Mount the disk in guest and keep the disk writing data continuously during 
block-mirror,
the file-system went to read-only after block-mirror.

2) This commit 6cdbceb12cf[mirror: Add filter-node-name to blockdev-mirror] 
introduces
mirror_top_bs which does not set default function for 
mirror_top_bs->drv->bdrv_co_ioctl.

3) The function bdrv_co_ioctl in block/io.c will be called during block-mirror, 
in this
function, the judgment is as follow:
---
    if (!drv || (!drv->bdrv_aio_ioctl && !drv->bdrv_co_ioctl)) {
        co.ret = -ENOTSUP;
        goto out;
    }
---
The mirror_top_bs does not set drv->bdrv_aio_ioctl or drv->bdrv_co_ioctl which 
result this
return -ENOTSUP. So the file-system went to read-only after block-mirror.

4) This patch set a default function for mirror_top_bs->drv->bdrv_aio_ioctl, 
fix this problem.
---
 block/mirror.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 019f6deaa5..63b788ec39 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1480,6 +1480,12 @@ static int coroutine_fn 
bdrv_mirror_top_flush(BlockDriverState *bs)
     return bdrv_co_flush(bs->backing->bs);
 }
 
+static int coroutine_fn bdrv_mirror_top_ioctl(BlockDriverState *bs,
+    unsigned long int req, void *buf)
+{
+    return 0;
+}
+
 static int coroutine_fn bdrv_mirror_top_pwrite_zeroes(BlockDriverState *bs,
     int64_t offset, int bytes, BdrvRequestFlags flags)
 {
@@ -1555,6 +1561,7 @@ static BlockDriver bdrv_mirror_top = {
     .bdrv_co_pwrite_zeroes      = bdrv_mirror_top_pwrite_zeroes,
     .bdrv_co_pdiscard           = bdrv_mirror_top_pdiscard,
     .bdrv_co_flush              = bdrv_mirror_top_flush,
+    .bdrv_co_ioctl              = bdrv_mirror_top_ioctl,
     .bdrv_refresh_filename      = bdrv_mirror_top_refresh_filename,
     .bdrv_child_perm            = bdrv_mirror_top_child_perm,
 
-- 
2.27.0