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

From: Alexander Graf
Subject: Re: [Qemu-ppc] [RFC PATCH] file ram alloc: fail if cannot preallocate
Date: Fri, 21 Feb 2014 14:10:49 +0100

On 21.02.2014, at 14:04, Alexey Kardashevskiy <address@hidden> wrote:

> On 02/22/2014 12:02 AM, Alexander Graf wrote:
>> 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.
> 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.

Only if they're backed by virtual memory. And in that case why don't you want 
them be huge pages? What qualifies a region to be huge vs non-huge?


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