Re: [Qemu-devel] [PATCH v2 2/3] migration: use the free page reporting feature from balloon

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

* Wei Wang (wei.w.wang@intel.com) wrote:
> Use the free page reporting feature from the balloon device to clear the
> bits corresponding to guest free pages from the dirty bitmap, so that the
> free memory are not sent.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> CC: Michael S. Tsirkin <mst@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> ---
>  migration/ram.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index d6f462c..4fe16d2 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -49,6 +49,7 @@
>  #include "qemu/rcu_queue.h"
>  #include "migration/colo.h"
>  #include "migration/block.h"
> +#include "sysemu/balloon.h"
>  
>  /***********************************************************/
>  /* ram save/restore */
> @@ -206,6 +207,10 @@ struct RAMState {
>      uint32_t last_version;
>      /* We are in the first round */
>      bool ram_bulk_stage;
> +    /* The feature, skipping the transfer of free pages, is supported */
> +    bool free_page_support;
> +    /* Skip the transfer of free pages in the bulk stage */
> +    bool free_page_done;
>      /* How many times we have dirty too many pages */
>      int dirty_rate_high_cnt;
>      /* these variables are used for bitmap sync */
> @@ -773,7 +778,7 @@ unsigned long migration_bitmap_find_dirty(RAMState *rs, RAMBlock *rb,
>      unsigned long *bitmap = rb->bmap;
>      unsigned long next;
>  
> -    if (rs->ram_bulk_stage && start > 0) {
> +    if (rs->ram_bulk_stage && start > 0 && !rs->free_page_support) {
>          next = start + 1;
>      } else {
>          next = find_next_bit(bitmap, size, start);
> @@ -1653,6 +1658,8 @@ static void ram_state_reset(RAMState *rs)
>      rs->last_page = 0;
>      rs->last_version = ram_list.version;
>      rs->ram_bulk_stage = true;
> +    rs->free_page_support = balloon_free_page_support();
> +    rs->free_page_done = false;
>  }
>  
>  #define MAX_WAIT 50 /* ms, half buffered_file limit */
> @@ -2135,7 +2142,7 @@ static int ram_state_init(RAMState **rsp)
>      return 0;
>  }
>  
> -static void ram_list_init_bitmaps(void)
> +static void ram_list_init_bitmaps(RAMState *rs)
>  {
>      RAMBlock *block;
>      unsigned long pages;
> @@ -2145,7 +2152,11 @@ static void ram_list_init_bitmaps(void)
>          QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
>              pages = block->max_length >> TARGET_PAGE_BITS;
>              block->bmap = bitmap_new(pages);
> -            bitmap_set(block->bmap, 0, pages);
> +            if (rs->free_page_support) {
> +                bitmap_set(block->bmap, 1, pages);

I don't understand how it makes sense to do that here;
ignoring anything ese it means that migration_dirty_pages is wrong
which could end up with migration finishing before all real pages are
sent.

Dave

> +            } else {
> +                bitmap_set(block->bmap, 0, pages);
> +            }
>              if (migrate_postcopy_ram()) {
>                  block->unsentmap = bitmap_new(pages);
>                  bitmap_set(block->unsentmap, 0, pages);
> @@ -2161,7 +2172,7 @@ static void ram_init_bitmaps(RAMState *rs)
>      qemu_mutex_lock_ramlist();
>      rcu_read_lock();
>  
> -    ram_list_init_bitmaps();
> +    ram_list_init_bitmaps(rs);
>      memory_global_dirty_log_start();
>      migration_bitmap_sync(rs);
>  
> @@ -2275,6 +2286,11 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>  
>      ram_control_before_iterate(f, RAM_CONTROL_ROUND);
>  
> +    if (rs->free_page_support && !rs->free_page_done) {
> +        balloon_free_page_poll();
> +        rs->free_page_done = true;
> +    }
> +
>      t0 = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      i = 0;
>      while ((ret = qemu_file_rate_limit(f)) == 0) {
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK