[PATCH 04/12] libxenguest: avoid allocating unused deferred-pages bitmap

[Jan Beulich <jbeulich@xxxxxxxx>]

Like for the dirty bitmap, it is unnecessary to allocate the deferred-
pages bitmap when all that's ever going to happen is a single all-dirty
run.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The clearing of the bitmap at the end of suspend_and_send_dirty() also
looks unnecessary - am I overlooking anything?

--- a/tools/libs/guest/xg_sr_save.c
+++ b/tools/libs/guest/xg_sr_save.c
@@ -130,7 +130,7 @@ static int write_batch(struct xc_sr_cont
                                                       ctx->save.batch_pfns[i]);
 
         /* Likely a ballooned page. */
-        if ( mfns[i] == INVALID_MFN )
+        if ( mfns[i] == INVALID_MFN && ctx->save.deferred_pages )
         {
             set_bit(ctx->save.batch_pfns[i], ctx->save.deferred_pages);
             ++ctx->save.nr_deferred_pages;
@@ -196,8 +196,12 @@ static int write_batch(struct xc_sr_cont
             {
                 if ( rc == -1 && errno == EAGAIN )
                 {
-                    set_bit(ctx->save.batch_pfns[i], ctx->save.deferred_pages);
-                    ++ctx->save.nr_deferred_pages;
+                    if ( ctx->save.deferred_pages )
+                    {
+                        set_bit(ctx->save.batch_pfns[i],
+                                ctx->save.deferred_pages);
+                        ++ctx->save.nr_deferred_pages;
+                    }
                     types[i] = XEN_DOMCTL_PFINFO_XTAB;
                     --nr_pages;
                 }
@@ -665,7 +669,8 @@ static int suspend_and_send_dirty(struct
     else
         xc_set_progress_prefix(xch, "Checkpointed save");
 
-    bitmap_or(dirty_bitmap, ctx->save.deferred_pages, ctx->save.p2m_size);
+    if ( ctx->save.deferred_pages )
+        bitmap_or(dirty_bitmap, ctx->save.deferred_pages, ctx->save.p2m_size);
 
     if ( !ctx->save.live && ctx->stream_type == XC_STREAM_COLO )
     {
@@ -682,7 +687,8 @@ static int suspend_and_send_dirty(struct
     if ( rc )
         goto out;
 
-    bitmap_clear(ctx->save.deferred_pages, ctx->save.p2m_size);
+    if ( ctx->save.deferred_pages )
+        bitmap_clear(ctx->save.deferred_pages, ctx->save.p2m_size);
     ctx->save.nr_deferred_pages = 0;
 
  out:
@@ -791,24 +797,31 @@ static int setup(struct xc_sr_context *c
 {
     xc_interface *xch = ctx->xch;
     int rc;
-    DECLARE_HYPERCALL_BUFFER_SHADOW(unsigned long, dirty_bitmap,
-                                    &ctx->save.dirty_bitmap_hbuf);
 
     rc = ctx->save.ops.setup(ctx);
     if ( rc )
         goto err;
 
-    dirty_bitmap = ctx->save.live || ctx->stream_type != XC_STREAM_PLAIN
-        ? xc_hypercall_buffer_alloc_pages(
-              xch, dirty_bitmap, NRPAGES(bitmap_size(ctx->save.p2m_size)))
-        : (void *)-1L;
+    if ( ctx->save.live || ctx->stream_type != XC_STREAM_PLAIN )
+    {
+        DECLARE_HYPERCALL_BUFFER_SHADOW(unsigned long, dirty_bitmap,
+                                        &ctx->save.dirty_bitmap_hbuf);
+
+        dirty_bitmap =
+            xc_hypercall_buffer_alloc_pages(
+                xch, dirty_bitmap, NRPAGES(bitmap_size(ctx->save.p2m_size)));
+        ctx->save.deferred_pages = bitmap_alloc(ctx->save.p2m_size);
+
+        if ( !dirty_bitmap || !ctx->save.deferred_pages )
+            goto enomem;
+    }
 
     ctx->save.batch_pfns = malloc(MAX_BATCH_SIZE *
                                   sizeof(*ctx->save.batch_pfns));
-    ctx->save.deferred_pages = bitmap_alloc(ctx->save.p2m_size);
 
-    if ( !ctx->save.batch_pfns || !dirty_bitmap || !ctx->save.deferred_pages )
+    if ( !ctx->save.batch_pfns )
     {
+    enomem:
         ERROR("Unable to allocate memory for dirty bitmaps, batch pfns and"
               " deferred pages");
         rc = -1;