Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed

[Avi Kivity]

Anthony Liguori wrote:
> The second concern is that if a guest can allocate arbitrary amounts 
> of memory, it could generate a swap storm.  Unfortunately, AFAIK, 
> Linux is not yet to a point where it can deal with swap fairness.  
> Hopefully this is a limitation that the IO controller folks are taking 
> into account.

Even if Linux was 100% fair (or rather 100% controlled -- we don't want 
fairness here) in dealing with memory overcommit, we don't want swapping 
just because a guest issued large requests.  Breaking up large requests 
will reduce efficiency (I believe negligibly is the limit is 1MB or 
above), swapping will be much worse.

And again, there's no real reason to allocate memory here, we should be 
doing I/O directly to the guest's scatter/gather buffers.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.