Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification

[Avi Kivity]

 On 10/11/2010 05:49 PM, Anthony Liguori wrote:
> On 10/11/2010 09:58 AM, Avi Kivity wrote:
> > > A leak is unacceptable.  It means an image can grow to an unbounded 
> > > size.  If you are a server provider offering multitenancy, then a 
> > > malicious guest can potentially grow the image beyond it's allotted 
> > > size causing a Denial of Service attack against another tenant.
> >
> >
> > This particular leak cannot grow, and is not controlled by the guest.
>
> As the image gets moved from hypervisor to hypervisor, it can keep 
> growing if given a chance to fill up the disk, then trim it all way.
>
> In a mixed hypervisor environment, it just becomes a numbers game.

I don't see how it can grow.  Both the freelist and the clusters it 
points to consume space, which becomes a leak once you move it to a 
hypervisor that doesn't understand the freelist.  The older hypervisor 
then allocates new blocks.  As soon as it performs a metadata scan (if 
ever), the freelist is reclaimed.

You could only get a growing leak if you moved it to a hypervisor that 
doesn't perform metadata scans, but then that is independent of the 
freelist.

> > > A freelist has to be a non-optional feature.  When the freelist bit 
> > > is set, an older QEMU cannot read the image.  If the freelist is 
> > > completed used, the freelist bit can be cleared and the image is 
> > > then usable by older QEMUs.
> >
> > Once we support TRIM (or detect zeros) we'll never have a clean 
> > freelist.
>
> Zero detection doesn't add to the free list.

Why not?  If a cluster is zero filled, you may drop it (assuming no 
backing image).

> A potential solution here is to treat TRIM a little differently than 
> we've been discussing.
>
> When TRIM happens, don't immediately write an unallocated cluster 
> entry for the L2.  Leave the L2 entry in-tact.  Don't actually write a 
> UCE to the L2 until you actually allocate the block.
>
> This implies a cost because you'll need to do metadata syncs to make 
> this work.  However, that eliminates leakage.

The information is lost on shutdown; and you can have a large number of 
unallocated-in-waiting clusters (like a TRIM issued by mkfs, or a user 
expecting a visit from RIAA).

A slight twist on your proposal is to have an allocated-but-may-drop bit 
in a L2 entry.  TRIM or zero detection sets the bit (leaving the cluster 
number intact).  A following write to the cluster needs to clear the 
bit; if we reallocate the cluster we need to replace it with a ZCE.

This makes the freelist all L2 entries with the bit set; it may be less 
efficient than a custom data structure though.

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