savevm-async: keep more free space when entering final stage

In qemu-server, we already allocate 2 * $mem_size + 500 MiB for driver
state (which was 32 MiB long ago according to git history). It seems
likely that the 30 MiB cutoff in the savevm-async implementation was
chosen based on that.

In bug #4476 [0], another issue caused the iteration to not make any
progress and the state file filled up all the way to the 30 MiB +
pending_size cutoff. Since the guest is not stopped immediately after
the check, it can still dirty some RAM and the current cutoff is not
enough for a reproducer VM (was done while bug #4476 still was not
fixed), dirtying memory with
> stress-ng -B 2 --bigheap-growth 64.0M'
After entering the final stage, savevm actually filled up the state
file completely, leading to an I/O error. It's probably the same
scenario as reported in the bug report, the error message was fixed in
commit a020815 ("savevm-async: fix function name in error message")
after the bug report.

If not for the bug, the cutoff will only be reached by a VM that's
dirtying RAM faster than can be written to the storage, so increase
the cutoff to 100 MiB to have a bigger chance to finish successfully,
while still trying to not increase downtime too much for
non-hibernation snapshots.

[0]: https://bugzilla.proxmox.com/show_bug.cgi?id=4476

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

debian/patches/pve/0017-PVE-add-savevm-async-for-background-state-snapshots.patch

@@ -33,13 +33,13 @@ Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
  include/migration/snapshot.h |   2 +
  include/monitor/hmp.h        |   5 +
  migration/meson.build        |   1 +
- migration/savevm-async.c     | 532 +++++++++++++++++++++++++++++++++++
+ migration/savevm-async.c     | 538 +++++++++++++++++++++++++++++++++++
  monitor/hmp-cmds.c           |  57 ++++
  qapi/migration.json          |  34 +++
  qapi/misc.json               |  32 +++
  qemu-options.hx              |  12 +
  softmmu/vl.c                 |  10 +
- 11 files changed, 731 insertions(+)
+ 11 files changed, 737 insertions(+)
  create mode 100644 migration/savevm-async.c
 
 diff --git a/hmp-commands-info.hx b/hmp-commands-info.hx
@@ -155,10 +155,10 @@ index 8cac83c06c..0842d00cd2 100644
  ), gnutls)
 diff --git a/migration/savevm-async.c b/migration/savevm-async.c
 new file mode 100644
-index 0000000000..4a4e91a26d
+index 0000000000..dc30558713
 --- /dev/null
 +++ b/migration/savevm-async.c
-@@ -0,0 +1,532 @@
+@@ -0,0 +1,538 @@
 +#include "qemu/osdep.h"
 +#include "migration/channel-savevm-async.h"
 +#include "migration/migration.h"
@@ -414,7 +414,13 @@ index 0000000000..4a4e91a26d
 +
 +        pending_size = pend_precopy + pend_compatible + pend_postcopy;
 +
-+        maxlen = blk_getlength(snap_state.target) - 30*1024*1024;
++        /*
++         * A guest reaching this cutoff is dirtying lots of RAM. It should be
++         * large enough so that the guest can't dirty this much between the
++         * check and the guest actually being stopped, but it should be small
++         * enough to avoid long downtimes for non-hibernation snapshots.
++         */
++        maxlen = blk_getlength(snap_state.target) - 100*1024*1024;
 +
 +        /* Note that there is no progress for pend_postcopy when iterating */
 +        if (pending_size - pend_postcopy > 400000 && snap_state.bs_pos + pending_size < maxlen) {

debian/patches/pve/0018-PVE-add-optional-buffer-size-to-QEMUFile.patch

@@ -192,10 +192,10 @@ index fa13d04d78..914f1a63a8 100644
  int qemu_fclose(QEMUFile *f);
  
 diff --git a/migration/savevm-async.c b/migration/savevm-async.c
-index 4a4e91a26d..20aae335ca 100644
+index dc30558713..a38e7351c1 100644
 --- a/migration/savevm-async.c
 +++ b/migration/savevm-async.c
-@@ -368,7 +368,7 @@ void qmp_savevm_start(bool has_statefile, const char *statefile, Error **errp)
+@@ -374,7 +374,7 @@ void qmp_savevm_start(bool has_statefile, const char *statefile, Error **errp)
  
      QIOChannel *ioc = QIO_CHANNEL(qio_channel_savevm_async_new(snap_state.target,
                                                                 &snap_state.bs_pos));
@@ -204,7 +204,7 @@ index 4a4e91a26d..20aae335ca 100644
  
      if (!snap_state.file) {
          error_set(errp, ERROR_CLASS_GENERIC_ERROR, "failed to open '%s'", statefile);
-@@ -501,7 +501,8 @@ int load_snapshot_from_blockdev(const char *filename, Error **errp)
+@@ -507,7 +507,8 @@ int load_snapshot_from_blockdev(const char *filename, Error **errp)
      blk_op_block_all(be, blocker);
  
      /* restore the VM state */

debian/patches/pve/0049-PVE-savevm-async-register-yank-before-migration_inco.patch

@@ -11,7 +11,7 @@ Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
  1 file changed, 5 insertions(+)
 
 diff --git a/migration/savevm-async.c b/migration/savevm-async.c
-index 20aae335ca..94c5ae1c81 100644
+index a38e7351c1..0b1b60c6ae 100644
 --- a/migration/savevm-async.c
 +++ b/migration/savevm-async.c
 @@ -20,6 +20,7 @@
@@ -22,7 +22,7 @@ index 20aae335ca..94c5ae1c81 100644
  
  /* #define DEBUG_SAVEVM_STATE */
  
-@@ -515,6 +516,10 @@ int load_snapshot_from_blockdev(const char *filename, Error **errp)
+@@ -521,6 +522,10 @@ int load_snapshot_from_blockdev(const char *filename, Error **errp)
      dirty_bitmap_mig_before_vm_start();
  
      qemu_fclose(f);