[PATCH v6 06/19] migration/multifd: Make MultiFDPages_t:offset a flexible array member

[Fabiano Rosas <farosas@suse.de>]

We're about to use MultiFDPages_t from inside the MultiFDSendData
payload union, which means we cannot have pointers to allocated data
inside the pages structure, otherwise we'd lose the reference to that
memory once another payload type touches the union. Move the offset
array into the end of the structure and turn it into a flexible array
member, so it is allocated along with the rest of MultiFDSendData in
the next patches.

Note that other pointers, such as the ramblock pointer are still fine
as long as the storage for them is not owned by the migration code and
can be correctly released at some point.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/multifd.c | 19 ++++++++++++-------
 migration/multifd.h |  4 ++--
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index 640e4450ff..717e71f539 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -98,6 +98,17 @@ struct {
     MultiFDMethods *ops;
 } *multifd_recv_state;
 
+static size_t multifd_ram_payload_size(void)
+{
+    uint32_t n = multifd_ram_page_count();
+
+    /*
+     * We keep an array of page offsets at the end of MultiFDPages_t,
+     * add space for it in the allocation.
+     */
+    return sizeof(MultiFDPages_t) + n * sizeof(ram_addr_t);
+}
+
 static bool multifd_use_packets(void)
 {
     return !migrate_mapped_ram();
@@ -394,18 +405,12 @@ static int multifd_recv_initial_packet(QIOChannel *c, Error **errp)
 
 static MultiFDPages_t *multifd_pages_init(uint32_t n)
 {
-    MultiFDPages_t *pages = g_new0(MultiFDPages_t, 1);
-
-    pages->offset = g_new0(ram_addr_t, n);
-
-    return pages;
+    return g_malloc0(multifd_ram_payload_size());
 }
 
 static void multifd_pages_clear(MultiFDPages_t *pages)
 {
     multifd_pages_reset(pages);
-    g_free(pages->offset);
-    pages->offset = NULL;
     g_free(pages);
 }
 
diff --git a/migration/multifd.h b/migration/multifd.h
index 7bb4a2cbc4..a7fdd97f70 100644
--- a/migration/multifd.h
+++ b/migration/multifd.h
@@ -77,9 +77,9 @@ typedef struct {
     uint32_t num;
     /* number of normal pages */
     uint32_t normal_num;
+    RAMBlock *block;
     /* offset of each page */
-    ram_addr_t *offset;
-    RAMBlock *block;
+    ram_addr_t offset[];
 } MultiFDPages_t;
 
 struct MultiFDRecvData {
-- 
2.35.3