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Re: [Xen-devel] [PATCH v2] fix Remus failover regression



Hi Andrew,

On 07/28/2014 05:24 PM, Andrew Cooper wrote:
On 28/07/14 05:03, Yang Hongyang wrote:
commit: c2ba706c
tools/libxc: goto correct label on error paths by Andrew Cooper
broke Remus in Xen 4.4 or earlier versions that has this commit
backported.

My appologies for breaking Remus. (it just goes to show how fragile this
code is).


With Remus, this jump essentially discards the current incomplete
checkpoint received by the backup and restore backup from the
last complete checkpoint.
This is required for Remus to work and this does not break live
migration.
It has been around since Xen 4.0.

However, it is a genuine bugfix for regular migration, so simply
reverting it as this patch does is not appropriate.

For regular migration, you absolutely have to goto out; on a failure
otherwise the finish code will run and declare the migration a success
despite only having half a domain restored.

I think regular migration shouldn't run into this path (see what I commented
in v1), but I agree that add a check will be better.


You need something like:

if ( !checkpointed_stream )
     goto err;

/* Remus comment */
goto finish;

to deal with the different error handing requirements of remus and
regular streams.

~Andrew


CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Shriram Rajagopalan <rshriram@xxxxxxxxx>
Signed-off-by: Yang Hongyang <yanghy@xxxxxxxxxxxxxx>
---
  tools/libxc/xc_domain_restore.c | 13 +++++++++++--
  1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/tools/libxc/xc_domain_restore.c b/tools/libxc/xc_domain_restore.c
index e73e0a2..b9a56d5 100644
--- a/tools/libxc/xc_domain_restore.c
+++ b/tools/libxc/xc_domain_restore.c
@@ -1783,20 +1783,29 @@ int xc_domain_restore(xc_interface *xch, int io_fd, 
uint32_t dom,

      if ( pagebuf_get(xch, ctx, &pagebuf, io_fd, dom) ) {
          PERROR("error when buffering batch, finishing");
-        goto out;
+        /*
+         * Remus: discard the current incomplete checkpoint and restore
+         * backup from the last complete checkpoint.
+         */
+        goto finish;
      }
      memset(&tmptail, 0, sizeof(tmptail));
      tmptail.ishvm = hvm;
      if ( buffer_tail(xch, ctx, &tmptail, io_fd, max_vcpu_id, vcpumap,
                       ext_vcpucontext, vcpuextstate_size) < 0 ) {
          ERROR ("error buffering image tail, finishing");
-        goto out;
+        /*
+         * Remus: discard the current incomplete checkpoint and restore
+         * backup from the last complete checkpoint.
+         */
+        goto finish;
      }
      tailbuf_free(&tailbuf);
      memcpy(&tailbuf, &tmptail, sizeof(tailbuf));

      goto loadpages;

+  /* With Remus: restore from last complete checkpoint */
    finish:
      if ( hvm )
          goto finish_hvm;

.


--
Thanks,
Yang.

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