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

[Alexander Graf]

On 21.02.2014, at 13:56, Alexey Kardashevskiy <address@hidden> wrote:

> On 02/21/2014 07:57 PM, Alexander Graf wrote:
>> 
>> On 21.02.2014, at 05:57, Alexey Kardashevskiy <address@hidden> 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.

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


Alex