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




On 10/25/2013 10:44 PM, Mike Snitzer wrote:
On Fri, Oct 25 2013 at 12:37pm -0400,
Alasdair G Kergon <agk redhat com> wrote:

On Thu, Oct 24, 2013 at 02:30:19PM -0400, Mike Snitzer wrote:
From: Heinz Mauelshagen <heinzm 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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