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Re: [PATCH v13 0/5] UFFD write-tracking migration/snapshots

From: Peter Xu
Subject: Re: [PATCH v13 0/5] UFFD write-tracking migration/snapshots
Date: Mon, 22 Feb 2021 12:29:23 -0500

On Sat, Feb 20, 2021 at 02:59:42AM -0500, David Hildenbrand wrote:
> Live snapshotting ends up reading all guest memory (dirty bitmap starts with 
> all 1s), which is not what we want for virtio-mem - we don’t want to read and 
> migrate memory that has been discarded and has no stable content.
> For ordinary migration we use the guest page hint API to clear bits in the 
> dirty bitmap after dirty bitmap sync. Well, if we don‘t do bitmap syncs we‘ll 
> never clear any dirty bits. That‘s the problem.

Using dirty bitmap for that information is less efficient, becase it's
definitely a larger granularity information than PAGE_SIZE.  If the disgarded
ranges are always continuous and at the end of a memory region, we should have
some parameter in the ramblock showing that where we got shrinked then we don't
check dirty bitmap at all, rather than always assuming used_length is the one.


Peter Xu

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