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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 09:57:59 +0100

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.

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"?

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


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