Re: [PATCH 6/7] migration: Fix arrays of pointers in JSON writer

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Tue, Jan 07, 2025 at 04:50:24PM -0300, Fabiano Rosas wrote:
>> Currently, if an array of pointers contains a NULL pointer, that
>> pointer will be encoded as '0' in the stream. Since the JSON writer
>> doesn't define a "pointer" type, that '0' will now be an uint64, which
>> is different from the original type being pointed to, e.g. struct.
>> 
>> That mixed-type array shouldn't be compressed, otherwise data is lost
>> as the code currently makes the whole array have the type of the first
>> element.
>> 
>> While we could disable the array compression when a NULL pointer is
>> found, the JSON part of the stream still makes part of downtime, so we
>> should avoid writing unecessary bytes to it.
>> 
>> Keep the array compression in place, but break the array into several
>> type-contiguous pieces if NULL and non-NULL pointers are mixed.
>
> Could I request for a sample JSON dump for an example array in the commit
> log?  This whole solution looks working but is tricky.  A sample could help
> people understand (e.g. showing the same "name" being dumped multiple
> times..).

{"name": "s390_css", "instance_id": 0, "vmsd_name": "s390_css",
 "version": 1, "fields": [
   ...,
   {"name": "css", "array_len": 254, "type": "uint8", "size": 1},
   {"name": "css", "type": "struct", "struct": {
    "vmsd_name": "s390_css_img", "version": 1, "fields": [{"name":
    "chpids", "array_len": 256, "type": "struct", "struct": {"vmsd_name":
    "s390_chp_info", "version": 1, "fields": [{"name": "in_use", "type":
    "uint8", "size": 1}, {"name": "type", "type": "uint8", "size": 1},
    {"name": "is_virtual", "type": "uint8", "size": 1}]}, "size": 3}]},
    "size": 768},
   {"name": "css", "type": "uint8", "size": 1},
   ...
]}

>
> Side note: I tried to dump a very basic VM's JSON out to disk, it scares me
> on the size:
>
> $ ls -lhS JSON.out 
> -rw-r--r--. 1 peterx peterx 106K Jan  7 17:18 JSON.out
>
> That's a simplest VM with all default stuff, mostly nothing complex.. I may
> really need to measure how the JSON debug strings affect migration function
> or perf at some point..
>

Agreed.

>> 
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>>  migration/vmstate.c          | 33 ++++++++++++++++++++++++++++++++-
>>  scripts/analyze-migration.py |  9 ++++++++-
>>  2 files changed, 40 insertions(+), 2 deletions(-)
>> 
>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>> index 52704c822c..a79ccf3875 100644
>> --- a/migration/vmstate.c
>> +++ b/migration/vmstate.c
>> @@ -425,15 +425,19 @@ int vmstate_save_state_v(QEMUFile *f, const 
>> VMStateDescription *vmsd,
>>              int size = vmstate_size(opaque, field);
>>              uint64_t old_offset, written_bytes;
>>              JSONWriter *vmdesc_loop = vmdesc;
>> +            bool is_prev_null = false;
>>  
>>              trace_vmstate_save_state_loop(vmsd->name, field->name, n_elems);
>>              if (field->flags & VMS_POINTER) {
>>                  first_elem = *(void **)first_elem;
>>                  assert(first_elem || !n_elems || !size);
>>              }
>> +
>>              for (i = 0; i < n_elems; i++) {
>>                  void *curr_elem = first_elem + size * i;
>>                  const VMStateField *inner_field;
>> +                bool is_null;
>> +                int max_elems = n_elems - i;
>>  
>>                  old_offset = qemu_file_transferred(f);
>>                  if (field->flags & VMS_ARRAY_OF_POINTER) {
>> @@ -448,12 +452,39 @@ int vmstate_save_state_v(QEMUFile *f, const 
>> VMStateDescription *vmsd,
>>                       * not follow.
>>                       */
>>                      inner_field = vmsd_create_fake_nullptr_field(field);
>> +                    is_null = true;
>>                  } else {
>>                      inner_field = field;
>> +                    is_null = false;
>> +                }
>> +
>> +                /*
>> +                 * Due to the fake nullptr handling above, if there's mixed
>> +                 * null/non-null data, it doesn't make sense to emit a
>> +                 * compressed array representation spanning the entire array
>> +                 * because the field types will be different (e.g. struct
>> +                 * vs. uint64_t). Search ahead for the next null/non-null
>> +                 * element and start a new compressed array if found.
>> +                 */
>> +                if (field->flags & VMS_ARRAY_OF_POINTER &&
>> +                    is_null != is_prev_null) {
>> +
>> +                    is_prev_null = is_null;
>> +                    vmdesc_loop = vmdesc;
>> +
>> +                    for (int j = i + 1; j < n_elems; j++) {
>> +                        void *elem = *(void **)(first_elem + size * j);
>> +                        bool elem_is_null = !elem && size;
>> +
>> +                        if (is_null != elem_is_null) {
>> +                            max_elems = j - i;
>> +                            break;
>> +                        }
>> +                    }
>>                  }
>>  
>>                  vmsd_desc_field_start(vmsd, vmdesc_loop, inner_field,
>> -                                      i, n_elems);
>> +                                      i, max_elems);
>>  
>>                  if (inner_field->flags & VMS_STRUCT) {
>>                      ret = vmstate_save_state(f, inner_field->vmsd,
>> diff --git a/scripts/analyze-migration.py b/scripts/analyze-migration.py
>> index 4836920ddc..9138e91a11 100755
>> --- a/scripts/analyze-migration.py
>> +++ b/scripts/analyze-migration.py
>> @@ -497,7 +497,14 @@ def read(self):
>>                      raise Exception("internal index of data field unmatched 
>> (%d/%d)" % (len(a), int(field['index'])))
>>                  a.append(field['data'])

There's actually a bug here, the code above does:

  if len(a) != int(field['index']):
      raise Exception()

Which only works with this patch because the compressed array happens to
come first.

>>              else:
>> -                self.data[field['name']] = field['data']
>> +                # There could be multiple entries for the same field
>> +                # name, e.g. when a compressed array was broken in
>> +                # more than one piece.
>> +                if (field['name'] in self.data and
>> +                    type(self.data[field['name']]) == list):
>> +                    self.data[field['name']].append(field['data'])
>> +                else:
>> +                    self.data[field['name']] = field['data']
>
> Do we realy need these script changes?  I thought VMSDFieldStruct always
> breaks array_len field into "index" based anyway?
>
>         new_fields = []
>         for field in self.desc['struct']['fields']:
>             if not 'array_len' in field:
>                 new_fields.append(field)
>                 continue
>             array_len = field.pop('array_len')
>             field['index'] = 0
>             new_fields.append(field)
>             for i in range(1, array_len):
>                 c = field.copy()
>                 c['index'] = i
>                 new_fields.append(c)
>
>         self.desc['struct']['fields'] = new_fields

This code is about decompressing the array, it doesn't handle multiple
entries with the same name. See the JSON I posted up there.

This makes the single:

  {"name": "css", "array_len": 254, "type": "uint8", "size": 1},

become multiple:

  {"name": "css", "index": 0, "type": "uint8", "size": 1},
  {"name": "css", "index": 1, "type": "uint8", "size": 1},
  ...
  {"name": "css", "index": 253, "type": "uint8", "size": 1},