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

[Peter Xu <peterx@redhat.com>]

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..).

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..

> 
> 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'])
>              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

-- 
Peter Xu