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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: Fri, 19 Feb 2021 16:10:54 -0500

On Fri, Feb 19, 2021 at 03:50:52PM -0500, Peter Xu wrote:
> Andrey,
> On Fri, Feb 19, 2021 at 09:57:37AM +0300, Andrey Gruzdev wrote:
> > For the discards that happen before snapshot is started, I need to dig into 
> > Linux and QEMU virtio-baloon
> > code more to get clear with it.
> Yes it's very tricky on how the error could trigger.
> Let's think of below sequence:
>   - Start a guest with init_on_free=1 set and also a virtio-balloon device
>   - Guest frees a page P and zeroed it (since init_on_free=1). Now P contains
>     all zeros.
>   - Virtio-balloon reports this page to host, MADV_DONTNEED sent, then this
>     page is dropped on the host.
>   - Start live snapshot, wr-protect all pages (but not including page P 
> because
>     it's currently missing).  Let's call it $SNAPSHOT1.
>   - Guest does alloc_page(__GFP_ZERO), accidentally fetching this page P and
>     returned
>   - So far, page P is still all zero (which is good!), then guest uses page P
>     and writes data to it (say, now P has data P1 rather than all zeros).
>   - Live snapshot saves page P, which content P1 rather than all zeros.
>   - Live snapshot completed.  Saved as $SNAPSHOT1.
> Then when load snapshot $SNAPSHOT1, we'll have P contains data P1.  After
> snapshot loaded, when guest allocate again with alloc_page(__GFP_ZERO) on this
> page P, since guest kernel "thought" this page is all-zero already so 
> memzero()
> is skipped even if __GFP_ZERO is provided.  Then this page P (with content P1)
> got returned for the alloc_page(__GFP_ZERO) even if __GFP_ZERO set.  That 
> could
> break the caller of alloc_page().
> > Anyhow I'm quite sure that adding global MISSING handler for snapshotting
> > is too heavy and not really needed.
> UFFDIO_ZEROCOPY installs a zero pfn and that should be all of it.  There'll
> definitely be overhead, but it may not be that huge as imagined.  Live 
> snapshot
> is great in that we have point-in-time image of guest without stopping the
> guest, so taking slightly longer time won't be a huge loss to us too.
> Actually we can also think of other ways to work around it.  One way is we can
> pre-fault all guest pages before wr-protect.  Note that we don't need to write
> to the guest page because read would suffice, since uffd-wp would also work
> with zero pfn.  It's just that this workaround won't help on saving snapshot
> disk space, but it seems working.  It would be great if you have other
> workarounds, maybe as you said UFFDIO_ZEROCOPY is not the only route.

Wait.. it actually seems to also solve the disk usage issue.. :)

We should just need to make sure to prohibit balloon before staring to
pre-fault read on all guest ram.  Seems awkward, but also seems working.. Hmm..

Peter Xu

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