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

[David Hildenbrand]

On 29.07.21 22:00, Peter Xu wrote:
> On Thu, Jul 29, 2021 at 09:39:24PM +0200, David Hildenbrand wrote:
> > > > 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.
>
> Good point, however.. then I'm wondering whether we should just init clear_bmap
> to all 1's too when init just like dirty bmap. :)

Yes, but ... I'm not sure if we have to get the dirty bits into 
KVMSlot->dirty_bmap as well in order to clear them.

It could work with "manual_dirty_log_protect". For 
!manual_dirty_log_protect we might have to keep it that way ... which 
means we might have to expose some ugly details up to migration/ram.c .
Might require some thought :)

> Besides: let's not be affected with these details as they should be more
> suitable to be handled separately; maybe I'll follow this up.  It could be a
> place to discuss things, but shouldn't be a burden to block this series.


Agreed :)


--
Thanks,

David / dhildenb