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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

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?


> This is also something that should be coordinated with the -mem-path refactoring.

Oh. There is always refactoring :(



-- 
Alexey