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

From: Peter Xu
Subject: Re: [PATCH v2 0/6] migration/ram: Optimize for virtio-mem via RamDiscardManager
Date: Thu, 29 Jul 2021 16:00:32 -0400

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. :)

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.


Peter Xu

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