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On 11/28/2011 07:04 PM, Jeff Mahoney wrote:
> On 11/28/2011 06:53 PM, Andi Kleen wrote:
>> Jeff Mahoney <jeffm@xxxxxxxx> writes:
>
>>> The extent_state structure is used at the core of the extent
>>> i/o code for managing flags, locking, etc. It requires
>>> allocations deep in the write code and if failures occur they
>>> are difficult to recover from.
>>>
>>> We avoid most of the failures by using a mempool, which can
>>> sleep when required, to honor the allocations. This allows
>>> future patches to convert most of the
>>> {set,clear,convert}_extent_bit and derivatives to return void.
>
>> Is this really safe?
>
>> iirc if there's any operation that needs multiple mempool
>> objects you can get into the following ABBA style deadlock:
>
>> thread 1 thread 2
>
>> get object A from pool 1 get object C from pool 2 get object B
>> from pool 2 pool 2 full -> block get object D from pool 2
> ^ pool1, i assume
>> pool1 full -> block
>
>
>> Now for thread 1 to make progress it needs thread 2 to free its
>> object first, but that needs thread 1 to free its object also
>> first, which is a deadlock.
>
>> It would still work if there are other users which eventually
>> make progress, but if you're really unlucky either pool 1 or 2
>> is complete used by threads doing a multiple object operation. So
>> you got a nasty rare deadlock ...
>
> Yes, I think you're right. I think the risk is probably there and
> I'm not sure it can be totally mitigated. We'd be stuck if there is
> a string of pathological cases and both mempool are empty. The only
> way I can see to try to help the situation is to make the mempool
> fairly substantial, which is what I did here. I'd prefer to make
> the mempools per-fs but that would require pretty heavy
> modifications in order to pass around a per-fs struct. In any case,
> the risk isn't totally eliminated.
The more I look into it, the more I don't think this is an uncommon
scenario in the kernel. Device mapper draws from a number of mempools
that can be interspersed with allocations from the bio pool. Even
without stacking different types of dm devices, I think we can run
into this scenario. The DM scenario is probably even worse since the
allocs are GFP_NOIO instead of the (relatively) more relaxed GFP_NOFS.
I'm not saying it's ok, just that there are similar sites already that
seem to be easier to hit but we haven't heard of issues there either.
It seems to be a behavior that is largely mitigated by establishing
mempools of sufficient size.
- -Jeff
- --
Jeff Mahoney
SUSE Labs
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