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 4:55 PM, Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
> * 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>
>
> Yes it seems to work, but there is a small bug,
> if we look at:
>
> static void ram_migration_cleanup(void *opaque)
> {
>     /* caller have hold iothread lock or is in a bh, so there is
>      * no writing race against this migration_bitmap
>      */
>     struct BitmapRcu *bitmap = migration_bitmap_rcu;
>     atomic_rcu_set(&migration_bitmap_rcu, NULL);
>     if (bitmap) {
>         memory_global_dirty_log_stop();
>         call_rcu(bitmap, migration_bitmap_free, rcu);
>     }
>
>
> we see it doesn't cause memory_global_dirty_log_stop
> if migration_bitmap_rcu is NULL; so we end up
> still calling dirty_log_start in ram_save_init but
> never call stop.
>

Exactly. This is what kept bugging me too, which is why I asked you on
the IRC the other day (if you vaguely remember) about whether I will
have to handle cleanup separately...Maybe I didn't frame it well that
day.

> I suggest you change it to:
>
>     migration_bitmap_rcu = g_new0(struct BitmapRcu, 1);
>     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);
>
> The cleanup routine still causes the dirty_log_stop() then.

Right, I will do that.

Ashijeet

> 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..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);
>>
>> -    if (migrate_postcopy_ram()) {
>> -        migration_bitmap_rcu->unsentmap = bitmap_new(ram_bitmap_pages);
>> -        bitmap_set(migration_bitmap_rcu->unsentmap, 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