Re: [PATCH v2 0/6] migration/ram: Optimize for virtio-mem via RamDiscardManager

[David Hildenbrand <david@redhat.com>]

>> In the meantime I adjusted the code but it does the clearing under the
>> iothread lock, which should not be what we want ... I'll have a look.
> 
> Thanks; if it takes more changes than expected we can still start from simple,
> IMHO, by taking bql and timely yield it.
> 
> At the meantime, I found two things in ram_init_bitmaps() that I'm not sure we
> need them of not:
> 
>    1. Do we need WITH_RCU_READ_LOCK_GUARD() if with both bql and ramlist lock?
>       (small question)

Good question, I'm not sure if we need it.

> 
>    2. Do we need migration_bitmap_sync_precopy() even if dirty bmap is all 1's?
>       (bigger question)

IIRC, the bitmap sync will fetch the proper dirty bitmap from KVM and 
set the proper bits in the clear_bitmap. So once we call 
migration_clear_memory_region_dirty_bitmap_range() etc. later we will 
actually clear dirty bits.

Without that, migration_clear_memory_region_dirty_bitmap_range() is a 
nop and we might migrate stuff unnecessarily twice as dirty bits are not 
cleared:

I certainly need that, otherwise the 
migration_clear_memory_region_dirty_bitmap_range() is a nop and it all 
won't work as you proposed.

-- 
Thanks,

David / dhildenb