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Re: [Qemu-devel] [PATCH] memory: do not add a reference to the owner of


From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] memory: do not add a reference to the owner of aliased regions
Date: Mon, 27 Jul 2015 18:11:20 +0300

On Mon, Jul 27, 2015 at 04:31:15PM +0200, Paolo Bonzini wrote:
> Very often the owner of the aliased region is the same as the owner of the 
> alias
> region itself.  When this happens, the reference count can never go back to 0 
> and
> the owner is leaked.  This is for example breaking hot-unplug of virtio-pci
> devices (the device cannot be plugged back again with the same id).
> 
> Another common use for alias is to transform the system I/O address space
> into an MMIO regions; in this case the aliased region never dies, so there
> is no problem.  Otherwise the owner is always the same for aliasing
> and aliased region.
> 
> I checked all calls to memory_region_init_alias introduced after commit
> dfde4e6 (memory: add ref/unref calls, 2013-05-06) and they do not need the
> reference in order to keep the owner of the aliased region alive.
> 
> Reported-by: Michael S. Tsirkin <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>


Tested-by: Michael S. Tsirkin <address@hidden>

> ---
>  memory.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/memory.c b/memory.c
> index 5e5f325..4eb138a 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -859,11 +859,6 @@ static void memory_region_destructor_ram(MemoryRegion 
> *mr)
>      qemu_ram_free(mr->ram_addr);
>  }
>  
> -static void memory_region_destructor_alias(MemoryRegion *mr)
> -{
> -    memory_region_unref(mr->alias);
> -}
> -
>  static void memory_region_destructor_ram_from_ptr(MemoryRegion *mr)
>  {
>      qemu_ram_free_from_ptr(mr->ram_addr);
> @@ -1272,8 +1267,6 @@ void memory_region_init_alias(MemoryRegion *mr,
>                                uint64_t size)
>  {
>      memory_region_init(mr, owner, name, size);
> -    memory_region_ref(orig);
> -    mr->destructor = memory_region_destructor_alias;
>      mr->alias = orig;
>      mr->alias_offset = offset;
>  }
> -- 
> 2.4.3



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