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

[Avi Kivity <avi@redhat.com>]

Ryan Harper wrote:

 

>> There are two concerns with allowing the guest to alloc arbitrary 
>> amounts of memory.  The first is that QEMU is not written in such a way 
>> to be robust in the face of out-of-memory conditions so if we run out of 
>> VA space, an important malloc could fail and we'd fall over.
>>     
>
> That is an understandable concern and I don't want to make matters
> worse, even if the instability already exists in the code as-is.  I think
> I'd like to see this fail in practice before I'm really concerned.  For
> 64-bit builds, is the VA space an issue?
>
>   

VA space is virtually (groan) unlimited on x86_64.  But note that for 
every kilobyte of virtual address space you allocate, you consume half a 
byte in nonpagepable kernel memory.

>> 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.
>>     
>
> Sure, but as I mentioned, the amount of memory it can allocate can
> surely be controlled by the host system per-process ulimit no?
>   

So if the guest issues some large I/Os in parallel, we kill it?

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