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migration/vmstate: avoid per-element heap churn in vmsd ptr marker field

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For every NULL slot in a VMS_ARRAY_OF_POINTER (or every entry of a
dynamic array), the saver allocates a 1-element fake VMStateField via
g_new0 and frees it again right after the save. For arrays of
thousands of entries this is thousands of malloc/free pairs on the
hot save path.

Replace the heap-allocated marker with a stack-resident field
populated by an init helper. The caller passes a pointer to a local
VMStateField, the helper fills it in (still asserting the
precondition), and no g_free is needed.

Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20260518110112.21395-4-guobin@linux.alibaba.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Bin Guo
2026-05-18 19:01:07 +08:00
committed by Peter Xu
parent 22facc21ab
commit 36dec7c619
1 changed files with 16 additions and 25 deletions
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41
migration/vmstate.c
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@@ -59,29 +59,23 @@ vmstate_field_exists(const VMStateDescription *vmsd, const VMStateField *field,
* array of a VMS_ARRAY_OF_POINTER VMSD field. It's needed because we
* can't dereference the NULL pointer.
*/
static const VMStateField *
vmsd_create_ptr_marker_field(const VMStateField *field)
static void
vmsd_init_ptr_marker_field(VMStateField *fake, const VMStateField *field)
{
VMStateField *fake = g_new0(VMStateField, 1);
/* It can only happen on an array of pointers! */
assert(field->flags & VMS_ARRAY_OF_POINTER);
/* Some of fake's properties should match the original's */
fake->name = field->name;
fake->version_id = field->version_id;
/* Do not need "field_exists" check as it always exists */
fake->field_exists = NULL;
/* See vmstate_info_ptr_marker - use 1 byte to represent ptr status */
fake->size = 1;
fake->info = &vmstate_info_ptr_marker;
fake->flags = VMS_SINGLE;
/* All the rest fields shouldn't matter.. */
return (const VMStateField *)fake;
/* See vmstate_info_ptr_marker - 1 byte represents ptr status */
*fake = (VMStateField) {
.name = field->name,
.version_id = field->version_id,
/* Marker always exists, no field_exists callback needed */
.field_exists = NULL,
.size = 1,
.info = &vmstate_info_ptr_marker,
.flags = VMS_SINGLE,
/* All other fields stay zero-initialised */
};
}
static int vmstate_n_elems(void *opaque, const VMStateField *field)
@@ -676,6 +670,7 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd,
for (i = 0; i < n_elems; i++) {
void *curr_elem = first_elem + size * i;
const VMStateField *inner_field;
VMStateField marker_field;
/* maximum number of elements to compress in the JSON blob */
int max_elems = vmsd_can_compress(field) ? (n_elems - i) : 1;
bool use_marker_field, is_null = false;
@@ -689,7 +684,8 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd,
use_marker_field = use_dynamic_array || is_null;
if (use_marker_field) {
inner_field = vmsd_create_ptr_marker_field(field);
vmsd_init_ptr_marker_field(&marker_field, field);
inner_field = &marker_field;
} else {
inner_field = field;
}
@@ -726,11 +722,6 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd,
inner_field, vmdesc_loop,
i, max_elems, errp);
/* If we used a fake temp field.. free it now */
if (use_marker_field) {
g_clear_pointer((gpointer *)&inner_field, g_free);
}
if (!ok) {
goto out;
}