[dm-devel] [PATCH 06/24] dm cache policy mq: return NULL if mq->free list is empty in alloc_entry

[Heinz Mauelshagen]

On 10/25/2013 10:44 PM, Mike Snitzer wrote:
> On Fri, Oct 25 2013 at 12:37pm -0400,
> Alasdair G Kergon <agk at redhat.com> wrote:
>
>> On Thu, Oct 24, 2013 at 02:30:19PM -0400, Mike Snitzer wrote:
>>> From: Heinz Mauelshagen <heinzm at redhat.com>
>>> Addresses callers' (insert_in*cache()) requirement that alloc_entry()
>>> return NULL when an entry isn't able to be allocated.
>>   
>> What is the code path that leads to the requirement for this patch?
>>
>>> +++ b/drivers/md/dm-cache-policy-mq.c
>>>   static struct entry *alloc_entry(struct mq_policy *mq)
>>> +	struct entry *e = NULL;
>>>   
>>>   	if (mq->nr_entries_allocated >= mq->nr_entries) {
>>>   		BUG_ON(!list_empty(&mq->free));
>>>   		return NULL;
>>>   	}
>>>   
>>> -	e = list_entry(list_pop(&mq->free), struct entry, list);
>>> -	INIT_LIST_HEAD(&e->list);
>>> -	INIT_HLIST_NODE(&e->hlist);
>>> +	if (!list_empty(&mq->free)) {
>>> +		e = list_entry(list_pop(&mq->free), struct entry, list);
>>> +		INIT_LIST_HEAD(&e->list);
>>> +		INIT_HLIST_NODE(&e->hlist);
>>> +		mq->nr_entries_allocated++;
>>> +	}
>> In other words, under what circumstances is mq->nr_entries_allocated
>> less then mq->nr_entries, yet the mq->free list is empty?
>>
>> Is it better to apply a patch like this, or rather to fix whatever situation
>> leads to those circumstances?  Has the bug/race always been there or is it a
>> regression?
> Fair questions, Heinz should explain further.
>
> IIRC this change was needed as a prereq for the conversion of his
> out-of-tree "background" policy to a shim (layered ontop of mq).

Has nothing to do with that.

It is a fix for existing callers presuming that alloc_entry() could 
return NULL but the
callee did not handle this properly.


>
> So will drop for now, can revisit if/when the need is clearer (e.g. when
> background policy goes upstream).  TBD if we need it in the near-term,
> but will table this for now.  Dropping patch until more context from
> Heinz is provided.

Leave it in for the aforementioned reason.

Heinz

>
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