Re: [Qemu-devel] [PATCH v3] migrate: Introduce zero RAM checks to skip RAM migration

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

* Ashijeet Acharya (ashijeetacharya@gmail.com) wrote:
> Migration of a "none" machine with no RAM crashes abruptly as
> bitmap_new() fails and thus aborts. Instead place zero RAM checks at
> appropriate places to skip migration of RAM in this case and complete
> migration successfully for devices only.
> 
> Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>

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

> ---
> Changes in v3:
> - handle migration cleanup appropriately (Dave)
> Changes in v2:
> - try to migrate successfully by skipping RAM (Paolo, Greg)
> - drop the idea of erroring out and failing nicely
> ---
>  migration/ram.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index ef8fadf..faff5ab 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1325,6 +1325,11 @@ static int ram_find_and_save_block(QEMUFile *f, bool last_stage,
>      ram_addr_t dirty_ram_abs; /* Address of the start of the dirty page in
>                                   ram_addr_t space */
>  
> +    /* No dirty page as there is zero RAM */
> +    if (!ram_bytes_total()) {
> +        return pages;
> +    }
> +
>      pss.block = last_seen_block;
>      pss.offset = last_offset;
>      pss.complete_round = false;
> @@ -1912,14 +1917,17 @@ static int ram_save_init_globals(void)
>      bytes_transferred = 0;
>      reset_ram_globals();
>  
> -    ram_bitmap_pages = last_ram_offset() >> TARGET_PAGE_BITS;
>      migration_bitmap_rcu = g_new0(struct BitmapRcu, 1);
> -    migration_bitmap_rcu->bmap = bitmap_new(ram_bitmap_pages);
> -    bitmap_set(migration_bitmap_rcu->bmap, 0, ram_bitmap_pages);
> -
> -    if (migrate_postcopy_ram()) {
> -        migration_bitmap_rcu->unsentmap = bitmap_new(ram_bitmap_pages);
> -        bitmap_set(migration_bitmap_rcu->unsentmap, 0, ram_bitmap_pages);
> +    /* Skip setting bitmap if there is no RAM */
> +    if (ram_bytes_total()) {
> +        ram_bitmap_pages = last_ram_offset() >> TARGET_PAGE_BITS;
> +        migration_bitmap_rcu->bmap = bitmap_new(ram_bitmap_pages);
> +        bitmap_set(migration_bitmap_rcu->bmap, 0, ram_bitmap_pages);
> +
> +        if (migrate_postcopy_ram()) {
> +            migration_bitmap_rcu->unsentmap = bitmap_new(ram_bitmap_pages);
> +            bitmap_set(migration_bitmap_rcu->unsentmap, 0, ram_bitmap_pages);
> +        }
>      }
>  
>      /*
> -- 
> 2.6.2
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK