Re: [Qemu-devel] [PATCH v2] migrate: Introduce zero RAM checks to skip RAM migration

[Ashijeet Acharya <ashijeetacharya@gmail.com>]

On Wed, Feb 8, 2017 at 5:19 PM, Thomas Huth <thuth@redhat.com> wrote:
> On 03.02.2017 11:36, Ashijeet Acharya 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>
>> ---
>> 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..2f19566 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);
>> +    /* Skip setting bitmap if there is no RAM */
>> +    if (ram_bytes_total()) {
>> +        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);
>
> I guess the problem was here that ram_bitmap_pages was 0 ? So instead of
> checking "if (ram_bytes_total())", would it maybe be nicer to check for
> "if (ram_bitmap_pages)" instead?
>
I thought of using that first, but ram_bytes_total() works for both
the places? No?

Ashijeet

>  Thomas
>