migration/postcopy: make PostcopyDiscardState a static variable

In postcopy-ram.c, we provide three functions to discard certain
RAMBlock range:

  * postcopy_discard_send_init()
  * postcopy_discard_send_range()
  * postcopy_discard_send_finish()

Currently, we allocate/deallocate PostcopyDiscardState for each RAMBlock
on sending discard information to destination. This is not necessary and
the same data area could be reused for each RAMBlock.

This patch defines PostcopyDiscardState a static variable. By doing so:

  1) avoid memory allocation and deallocation to the system
  2) avoid potential failure of memory allocation
  3) hide some details for their users

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Message-Id: <20190724010721.2146-1-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/postcopy-ram.c

@@ -1377,22 +1377,16 @@ void postcopy_fault_thread_notify(MigrationIncomingState *mis)
  *   asking to discard individual ranges.
  *
  * @ms: The current migration state.
- * @offset: the bitmap offset of the named RAMBlock in the migration
- *   bitmap.
+ * @offset: the bitmap offset of the named RAMBlock in the migration bitmap.
  * @name: RAMBlock that discards will operate on.
- *
- * returns: a new PDS.
  */
-PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
-                                                 const char *name)
+static PostcopyDiscardState pds = {0};
+void postcopy_discard_send_init(MigrationState *ms, const char *name)
 {
-    PostcopyDiscardState *res = g_malloc0(sizeof(PostcopyDiscardState));
-
-    if (res) {
-        res->ramblock_name = name;
-    }
-
-    return res;
+    pds.ramblock_name = name;
+    pds.cur_entry = 0;
+    pds.nsentwords = 0;
+    pds.nsentcmds = 0;
 }
 
 /**
@@ -1401,30 +1395,29 @@ PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
  *   be sent later.
  *
  * @ms: Current migration state.
- * @pds: Structure initialised by postcopy_discard_send_init().
  * @start,@length: a range of pages in the migration bitmap in the
  *   RAM block passed to postcopy_discard_send_init() (length=1 is one page)
  */
-void postcopy_discard_send_range(MigrationState *ms, PostcopyDiscardState *pds,
-                                unsigned long start, unsigned long length)
+void postcopy_discard_send_range(MigrationState *ms, unsigned long start,
+                                 unsigned long length)
 {
     size_t tp_size = qemu_target_page_size();
     /* Convert to byte offsets within the RAM block */
-    pds->start_list[pds->cur_entry] = start  * tp_size;
-    pds->length_list[pds->cur_entry] = length * tp_size;
-    trace_postcopy_discard_send_range(pds->ramblock_name, start, length);
-    pds->cur_entry++;
-    pds->nsentwords++;
+    pds.start_list[pds.cur_entry] = start  * tp_size;
+    pds.length_list[pds.cur_entry] = length * tp_size;
+    trace_postcopy_discard_send_range(pds.ramblock_name, start, length);
+    pds.cur_entry++;
+    pds.nsentwords++;
 
-    if (pds->cur_entry == MAX_DISCARDS_PER_COMMAND) {
+    if (pds.cur_entry == MAX_DISCARDS_PER_COMMAND) {
         /* Full set, ship it! */
         qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
-                                              pds->ramblock_name,
-                                              pds->cur_entry,
-                                              pds->start_list,
-                                              pds->length_list);
-        pds->nsentcmds++;
-        pds->cur_entry = 0;
+                                              pds.ramblock_name,
+                                              pds.cur_entry,
+                                              pds.start_list,
+                                              pds.length_list);
+        pds.nsentcmds++;
+        pds.cur_entry = 0;
     }
 }
 
@@ -1433,24 +1426,21 @@ void postcopy_discard_send_range(MigrationState *ms, PostcopyDiscardState *pds,
  * bitmap code. Sends any outstanding discard messages, frees the PDS
  *
  * @ms: Current migration state.
- * @pds: Structure initialised by postcopy_discard_send_init().
  */
-void postcopy_discard_send_finish(MigrationState *ms, PostcopyDiscardState *pds)
+void postcopy_discard_send_finish(MigrationState *ms)
 {
     /* Anything unsent? */
-    if (pds->cur_entry) {
+    if (pds.cur_entry) {
         qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
-                                              pds->ramblock_name,
-                                              pds->cur_entry,
-                                              pds->start_list,
-                                              pds->length_list);
-        pds->nsentcmds++;
+                                              pds.ramblock_name,
+                                              pds.cur_entry,
+                                              pds.start_list,
+                                              pds.length_list);
+        pds.nsentcmds++;
     }
 
-    trace_postcopy_discard_send_finish(pds->ramblock_name, pds->nsentwords,
-                                       pds->nsentcmds);
-
-    g_free(pds);
+    trace_postcopy_discard_send_finish(pds.ramblock_name, pds.nsentwords,
+                                       pds.nsentcmds);
 }
 
 /*

migration/postcopy-ram.h

@@ -43,10 +43,8 @@ int postcopy_ram_prepare_discard(MigrationIncomingState *mis);
 
 /*
  * Called at the start of each RAMBlock by the bitmap code.
- * Returns a new PDS
  */
-PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
-                                                 const char *name);
+void postcopy_discard_send_init(MigrationState *ms, const char *name);
 
 /*
  * Called by the bitmap code for each chunk to discard.
@@ -55,15 +53,14 @@ PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
  * @start,@length: a range of pages in the migration bitmap in the
  *  RAM block passed to postcopy_discard_send_init() (length=1 is one page)
  */
-void postcopy_discard_send_range(MigrationState *ms, PostcopyDiscardState *pds,
-                                 unsigned long start, unsigned long length);
+void postcopy_discard_send_range(MigrationState *ms, unsigned long start,
+                                 unsigned long length);
 
 /*
  * Called at the end of each RAMBlock by the bitmap code.
- * Sends any outstanding discard messages, frees the PDS.
+ * Sends any outstanding discard messages.
  */
-void postcopy_discard_send_finish(MigrationState *ms,
-                                  PostcopyDiscardState *pds);
+void postcopy_discard_send_finish(MigrationState *ms);
 
 /*
  * Place a page (from) at (host) efficiently

migration/ram.c

@@ -2850,12 +2850,9 @@ void ram_postcopy_migrated_memory_release(MigrationState *ms)
  *       with the dirtymap; so a '1' means it's either dirty or unsent.
  *
  * @ms: current migration state
- * @pds: state for postcopy
  * @block: RAMBlock to discard
  */
-static int postcopy_send_discard_bm_ram(MigrationState *ms,
-                                        PostcopyDiscardState *pds,
-                                        RAMBlock *block)
+static int postcopy_send_discard_bm_ram(MigrationState *ms, RAMBlock *block)
 {
     unsigned long end = block->used_length >> TARGET_PAGE_BITS;
     unsigned long current;
@@ -2876,7 +2873,7 @@ static int postcopy_send_discard_bm_ram(MigrationState *ms,
         } else {
             discard_length = zero - one;
         }
-        postcopy_discard_send_range(ms, pds, one, discard_length);
+        postcopy_discard_send_range(ms, one, discard_length);
         current = one + discard_length;
     }
 
@@ -2902,16 +2899,15 @@ static int postcopy_each_ram_send_discard(MigrationState *ms)
     int ret;
 
     RAMBLOCK_FOREACH_NOT_IGNORED(block) {
-        PostcopyDiscardState *pds =
-            postcopy_discard_send_init(ms, block->idstr);
+        postcopy_discard_send_init(ms, block->idstr);
 
         /*
          * Postcopy sends chunks of bitmap over the wire, but it
          * just needs indexes at this point, avoids it having
          * target page specific code.
          */
-        ret = postcopy_send_discard_bm_ram(ms, pds, block);
-        postcopy_discard_send_finish(ms, pds);
+        ret = postcopy_send_discard_bm_ram(ms, block);
+        postcopy_discard_send_finish(ms);
         if (ret) {
             return ret;
         }
@@ -2934,11 +2930,9 @@ static int postcopy_each_ram_send_discard(MigrationState *ms)
  * @unsent_pass: if true we need to canonicalize partially unsent host pages
  *               otherwise we need to canonicalize partially dirty host pages
  * @block: block that contains the page we want to canonicalize
- * @pds: state for postcopy
  */
 static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
-                                          RAMBlock *block,
-                                          PostcopyDiscardState *pds)
+                                          RAMBlock *block)
 {
     RAMState *rs = ram_state;
     unsigned long *bitmap = block->bmap;
@@ -3018,8 +3012,7 @@ static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
                  *     (any partially sent pages were already discarded
                  *     by the previous unsent_pass)
                  */
-                postcopy_discard_send_range(ms, pds, fixup_start_addr,
-                                            host_ratio);
+                postcopy_discard_send_range(ms, fixup_start_addr, host_ratio);
             }
 
             /* Clean up the bitmap */
@@ -3062,18 +3055,17 @@ static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
  */
 static int postcopy_chunk_hostpages(MigrationState *ms, RAMBlock *block)
 {
-    PostcopyDiscardState *pds =
-        postcopy_discard_send_init(ms, block->idstr);
+    postcopy_discard_send_init(ms, block->idstr);
 
     /* First pass: Discard all partially sent host pages */
-    postcopy_chunk_hostpages_pass(ms, true, block, pds);
+    postcopy_chunk_hostpages_pass(ms, true, block);
     /*
      * Second pass: Ensure that all partially dirty host pages are made
      * fully dirty.
      */
-    postcopy_chunk_hostpages_pass(ms, false, block, pds);
+    postcopy_chunk_hostpages_pass(ms, false, block);
 
-    postcopy_discard_send_finish(ms, pds);
+    postcopy_discard_send_finish(ms);
     return 0;
 }