[PATCH QEMU HCI-8.1 v2 21/24] VZ: migration/cpr-exec: abort cpr-exec if memory can't be preserved

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

cpr_exec_cb() marks all saved ramblocks with MADV_DOEXEC before exec, but
ignores the result.  If madvise() fails for some block, the walk stops
there and the remaining blocks are not marked, yet execvp() still runs.
The new QEMU then reuses addresses of guest RAM that wasn't preserved,
and the guest is lost.

Treat that as a failed cpr-exec: go to the same failure path as when
execvp() fails, so the VM keeps running in the old QEMU.

On that path also clear MADV_DOEXEC from all saved ramblocks again, just
like the preserved fds get close-on-exec back.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 migration/cpr-exec.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/migration/cpr-exec.c b/migration/cpr-exec.c
index 7b7609e19cd..cb962ce7252 100644
--- a/migration/cpr-exec.c
+++ b/migration/cpr-exec.c
@@ -154,6 +154,19 @@ static bool preserve_ram(cpr_ramblock_t ram, char *name)
     return true;
 }
 
+static bool unpreserve_ram(cpr_ramblock_t ram, char *name)
+{
+#ifdef MADV_DONTEXEC
+    qemu_madvise((void *) ram.addr, ram.length, MADV_DONTEXEC);
+#endif
+    return true;
+}
+
+static void cpr_exec_unpreserve_ram(void)
+{
+    cpr_walk_ramblock(unpreserve_ram);
+}
+
 static bool mark_ram_rdonly(cpr_ramblock_t ram, char *name)
 {
     /*
@@ -202,7 +215,10 @@ static void cpr_exec_cb(void *opaque)
      */
     cpr_exec_preserve_fds();
 
-    cpr_walk_ramblock(preserve_ram);
+    if (!cpr_walk_ramblock(preserve_ram)) {
+        /* preserve_ram() has already reported the error and failed migration */
+        goto fail;
+    }
     if (cpr_ram_rdonly) {
         cpr_walk_ramblock(mark_ram_rdonly);
     }
@@ -221,15 +237,17 @@ static void cpr_exec_cb(void *opaque)
      * or the system is very short on resources.
      */
     error_setg_errno(&err, errno, "execvp %s failed", argv[0]);
-    g_clear_pointer(&argv, g_strfreev);
-    cpr_exec_unpreserve_fds();
-
     error_report_err(error_copy(err));
     migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
     migrate_set_error(s, err);
     error_free(err);
     err = NULL;
 
+fail:
+    g_clear_pointer(&argv, g_strfreev);
+    cpr_exec_unpreserve_fds();
+    cpr_exec_unpreserve_ram();
+
     /* Note, we can go from state COMPLETED to FAILED */
     migration_call_notifiers(MIG_EVENT_PRECOPY_FAILED, NULL);
 

-- 
2.47.1