Re: [Qemu-devel] [PATCH] block: disable I/O limits at the beginning of bdrv_close()

[Alberto Garcia]

On Fri 25 Sep 2015 04:22:26 PM CEST, Eric Blake wrote:

>> Disabling I/O limits from a BDS also drains all pending throttled
>> requests, so it should be done at the beginning of bdrv_close() with
>> the rest of the bdrv_drain() calls before the BlockDriver is closed.
>
> Can this be abused? If I have a guest running in a cloud where the
> cloud provider has put severe throttling limits on me, but lets me
> hotplug to my heart's content, couldn't I just repeatedly plug/unplug
> the disk to get around the throttling (every time I unplug, all writes
> flush at full speed, then I immediately replug to start batching up a
> new set of writes).  In other words, shouldn't the draining still be
> throttled, to prevent my abuse?

I didn't think about this case, and I don't know how practical this is,
but note that bdrv_drain() (which is already at the beginning of
bdrv_close()) flushes the I/O queue explicitly bypassing the limits, so
other cases where a user can trigger a bdrv_drain() would also be
vulnerable to this.

Berto