Re: [Qemu-devel] [PATCH 40/40] memory: add reference counting to FlatView

[Jan Kiszka]

On 2013-05-07 20:00, Jan Kiszka wrote:
> On 2013-05-07 16:17, Paolo Bonzini wrote:
>> With this change, a FlatView can be used even after a concurrent
>> update has replaced it.  Because we do not have RCU, we use a
>> mutex to protect the small critical sections that read/write the
>> as->current_map pointer.  Accesses to the FlatView can be done
>> outside the mutex.
>>
>> If a MemoryRegion will be used after the FlatView is unref-ed (or after
>> a MemoryListener callback is returned), a reference has to be added to
>> that MemoryRegion.  For example, memory_region_find adds a reference to
>> the MemoryRegion that it returns.
> 
> For my understanding: Every lookup, e.g. triggered by address_space_rw,
> will briefly reference the FlatView of that address space and drop that
> reference again after referencing the target memory region.
> 
> Provided that is true: If we run that lookup on an address space that
> happens to be modified in parallel, the lookup may actually cause the
> final deref and, thus, release of the FlatView - even if the target
> memory region was totally unrelated to the ongoing change. That could
> make a hot-path pay the price of an action a slow path caused. Not
> really a beautiful concept. Even if the FlatView finalization is a
> simple free() (that is the minimum), we would pull the memory allocator
> into code paths that might try hard to keep a safe distance for the sake
> of predictability.

A mitigation could be to off-load the finalization to a bottom-half
e.g., ie. let the main iothread do the dirty work. Similar to the RCU
handler off-loading of the kernel. There is just a theoretical risk that
we pile up finalization requests and run out of memory. But all that is
pretty complicated and not really elegant. Better avoid the ref/unref
over hot-paths for objects of such broad scope like the FlatView.

Jan

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