Re: [Qemu-devel] [RFC PATCH] file ram alloc: fail if cannot preallocate

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 02/22/2014 12:02 AM, Alexander Graf wrote:
> 
> On 21.02.2014, at 13:56, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
>> On 02/21/2014 07:57 PM, Alexander Graf wrote:
>>>
>>> On 21.02.2014, at 05:57, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>>
>>>> On 02/10/2014 05:32 PM, Alexey Kardashevskiy wrote:
>>>>> At the moment if the user asked for huge pages and there is no more huge
>>>>> pages, QEMU prints warning and falls back to the anonymous memory
>>>>> allocator which is quite easy not to notice. QEMU also does so even
>>>>> if the user specified -mem-prealloc and it seems wrong as the user
>>>>> specifically requested huge pages for the entire RAM but QEMU failed to do
>>>>> so and continued. On PPC64 this will produce a fragile guest as QEMU
>>>>> tells the guest via device-tree that it can use huge pages when it
>>>>> actually cannot.
>>>>>
>>>>> This adds message+exit if RAM cannot be preallocated from huge pages.
>>>>
>>>>
>>>> Too bad? Should I increase my personal pinging timeout from 1 to 2 weeks to
>>>> avoid annoying the community? :) Thanks!
>>>
>>
>>> The patch changes the semantics of -mem-prealloc from "make sure all
>>> RAM is mapped" to "make sure all RAM is mapped and is backed by huge
>>> pages if we use huge pages" and thus is just plain wrong.
>>
>> ? I did actually expect it to alloc RAM from hugepages only. Otherwise
>> there is no point in mem-prealloc. Yes, I am ignorant, I know.
>>
>>> The real question is why are we allowing sparsely mapped huge page
>> backing at all? Should we change that? Do we need a new flag for this to
>> specify "yes, I do want all my pages backed by -mem-path"?
>>
>>
>> ? Add a switch to -mem-path saying "yes I really want -mem-path"? Sorry, I
>> lost you here. -mem-path + -mem-prealloc - like this is not enough? Why
>> would I specify -mem-path after all if I did not want RAM to backed by huge
>> pages?
> 

> I think it makes sense to disable any fallback for -mem-path, so that it
> always only allocates RAM pages from the -mem-path pool. But this is a
> big change from how it used to work before and thus needs to be properly
> coordinated.

ROMs, BARs - this all will stop working if I understand things right. And
we (ozlabs) do not really want these things to be in huge pages.

> 
> Paolo, Peter, any thoughts here? Version 2.0 might be a good fit for such a change ;).
> 
> 
> Alex
> 


-- 
Alexey