[Qemu-devel] [PATCH 1/1] Count used RAMBlock pages for migration_dirty_pages

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

This is a fix for a bug* triggered by a migration after hot unplugging
a few virtio-net NICs, that caused migration never to converge, because
'migration_dirty_pages' is incorrectly initialised.

'migration_dirty_pages' is used as a tally of the number of outstanding
dirty pages, to give the migration code an idea of how much more data
will need to be transferred, and thus whether it can end the iterative
phase.

It was initialised to the total size of the RAMBlock address space,
however hotunplug can leave this space sparse, and hence
migration_dirty_pages ended up too large.

Note that the code tries to be careful when counting to deal with
RAMBlocks that share the same end/start page - I don't know
if this is actually possible and it does complicate the code,
but since there was other code that dealt with unaligned RAMBlocks
it seemed possible.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

(* https://bugzilla.redhat.com/show_bug.cgi?id=1074913 )
---
 arch_init.c | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index f18f42e..ef0e98d 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -727,11 +727,8 @@ static void reset_ram_globals(void)
 static int ram_save_setup(QEMUFile *f, void *opaque)
 {
     RAMBlock *block;
-    int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS;
+    int64_t ram_bitmap_pages;
 
-    migration_bitmap = bitmap_new(ram_pages);
-    bitmap_set(migration_bitmap, 0, ram_pages);
-    migration_dirty_pages = ram_pages;
     mig_throttle_on = false;
     dirty_rate_high_cnt = 0;
 
@@ -770,6 +767,42 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     bytes_transferred = 0;
     reset_ram_globals();
 
+    ram_bitmap_pages = last_ram_offset() >> TARGET_PAGE_BITS;
+    migration_bitmap = bitmap_new(ram_bitmap_pages);
+    bitmap_set(migration_bitmap, 0, ram_bitmap_pages);
+    /*
+     * Count the total number of pages used by ram blocks. We clear the dirty
+     * bit for the start/end of each ramblock as we go so that we don't double
+     * count ramblocks that have overlapping pages - at entry the whole dirty
+     * bitmap is set.
+     */
+    migration_dirty_pages = 0;
+    QTAILQ_FOREACH(block, &ram_list.blocks, next) {
+        uint64_t block_pages = 0;
+        ram_addr_t saddr, eaddr;
+
+        saddr = block->mr->ram_addr;
+        eaddr = saddr + block->length - 1;
+        saddr /= TARGET_PAGE_SIZE;
+        eaddr /= TARGET_PAGE_SIZE;
+
+        /* Do the end pages of the range, that might be shared with others */
+        block_pages += test_and_clear_bit(saddr, migration_bitmap);
+        block_pages += test_and_clear_bit(eaddr, migration_bitmap);
+
+        if ((saddr + 1) < eaddr) {
+            block_pages += eaddr - (saddr + 1);
+        }
+        migration_dirty_pages += block_pages;
+        /*fprintf(stderr, "ram_save_setup: %s s/e=%zx/%zx page s/e=%zx/%zx"
+                " bp=%zu\n",
+                block->idstr, block->mr->ram_addr,
+                block->mr->ram_addr+block->length-1,
+                saddr, eaddr, block_pages); */
+    }
+    /* and set the bits again that got used for our overlap check */
+    bitmap_set(migration_bitmap, 0, ram_bitmap_pages);
+
     memory_global_dirty_log_start();
     migration_bitmap_sync();
     qemu_mutex_unlock_iothread();
-- 
1.8.5.3