On Fri, Oct 25 2013 at 12:37pm -0400,
Alasdair G Kergon <agk@xxxxxxxxxx> wrote:
> On Thu, Oct 24, 2013 at 02:30:19PM -0400, Mike Snitzer wrote:
> > From: Heinz Mauelshagen <heinzm@xxxxxxxxxx>
> > 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).
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.
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