[Qemu-devel] [PATCH v4 2/2] sheepdog: support 'qemu-img snapshot -a'

[Liu Yuan <namei.unix@gmail.com>]

Just call sd_create_branch() in the snapshot_goto to rollback the image is good
enough. With this patch, 'loadvm' process for sheepdog is modified:

Suppose we have a snapshot chain A --> B --> C, we do 'loadvm A' so as to get
a new chain,

A --> B
|
V
C1

in the old code:

1 reload inode of A (in snapshot_goto)
2 read vmstate via A's vdi_id (loadvm_state)
3 delete C and create C1, reload inode of C1 (sd_create_branch on write)

with this patch applied:

1 reload inode of A, delete C and create C1  (in snapshot_goto)
2 read vmstate via C1's parent, that is A's vdi_id (loadvm_state)

This will fix the possible bug that QEMU exit between 2 and 3 in the old code

Cc: qemu-devel@nongnu.org
Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 block/sheepdog.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/block/sheepdog.c b/block/sheepdog.c
index 94218ac..1b7c3f1 100644
--- a/block/sheepdog.c
+++ b/block/sheepdog.c
@@ -2071,14 +2071,11 @@ static int sd_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
         goto out;
     }
 
-    if (!s->inode.vm_state_size) {
-        error_report("Invalid snapshot");
-        ret = -ENOENT;
+    ret = sd_create_branch(s);
+    if (ret) {
         goto out;
     }
 
-    s->is_snapshot = true;
-
     g_free(old_s);
 
     return 0;
@@ -2196,8 +2193,9 @@ static int do_load_save_vmstate(BDRVSheepdogState *s, uint8_t *data,
     int fd, ret = 0, remaining = size;
     unsigned int data_len;
     uint64_t vmstate_oid;
-    uint32_t vdi_index;
     uint64_t offset;
+    uint32_t vdi_index;
+    uint32_t vdi_id = load ? s->inode.parent_vdi_id : s->inode.vdi_id;
 
     fd = connect_to_sdog(s);
     if (fd < 0) {
@@ -2210,7 +2208,7 @@ static int do_load_save_vmstate(BDRVSheepdogState *s, uint8_t *data,
 
         data_len = MIN(remaining, SD_DATA_OBJ_SIZE - offset);
 
-        vmstate_oid = vid_to_vmstate_oid(s->inode.vdi_id, vdi_index);
+        vmstate_oid = vid_to_vmstate_oid(vdi_id, vdi_index);
 
         create = (offset == 0);
         if (load) {
-- 
1.7.9.5