Re: [PATCH v1 06/10] powerpc/pseries/iommu: Add ddw_list_add() helper

Leonardo Bras
2020-08-27T22:11:38Z

On Mon, 2020-08-24 at 13:46 +1000, Alexey Kardashevskiy wrote:
> > static int find_existing_ddw_windows(void)
> > {
> > int len;
> > @@ -887,18 +905,11 @@ static int find_existing_ddw_windows(void)
> > if (!direct64)
> > continue;
> >
> > - window = kzalloc(sizeof(*window), GFP_KERNEL);
> > - if (!window || len < sizeof(struct dynamic_dma_window_prop)) {
> > + window = ddw_list_add(pdn, direct64);
> > + if (!window || len < sizeof(*direct64)) {
>
> Since you are touching this code, it looks like the "len <
> sizeof(*direct64)" part should go above to "if (!direct64)".

Sure, makes sense.
It will be fixed for v2.

>
>
>
> > kfree(window);
> > remove_ddw(pdn, true);
> > - continue;
> > }
> > -
> > - window->device = pdn;
> > - window->prop = direct64;
> > - spin_lock(&direct_window_list_lock);
> > - list_add(&window->list, &direct_window_list);
> > - spin_unlock(&direct_window_list_lock);
> > }
> >
> > return 0;
> > @@ -1261,7 +1272,8 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
> > dev_dbg(&dev->dev, "created tce table LIOBN 0x%x for %pOF\n",
> > create.liobn, dn);
> >
> > - window = kzalloc(sizeof(*window), GFP_KERNEL);
> > + /* Add new window to existing DDW list */
>
> The comment seems to duplicate what the ddw_list_add name already suggests.

Ok, I will remove it then.

> > + window = ddw_list_add(pdn, ddwprop);
> > if (!window)
> > goto out_clear_window;
> >
> > @@ -1280,16 +1292,14 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
> > goto out_free_window;
> > }
> >
> > - window->device = pdn;
> > - window->prop = ddwprop;
> > - spin_lock(&direct_window_list_lock);
> > - list_add(&window->list, &direct_window_list);
> > - spin_unlock(&direct_window_list_lock);
>
> I'd leave these 3 lines here and in find_existing_ddw_windows() (which
> would make ddw_list_add -> ddw_prop_alloc). In general you want to have
> less stuff to do on the failure path. kmalloc may fail and needs kfree
> but you can safely delay list_add (which cannot fail) and avoid having
> the lock help twice in the same function (one of them is hidden inside
> ddw_list_add).
> Not sure if this change is really needed after all. Thanks,

I understand this leads to better performance in case anything fails.
Also, I think list_add happening in the end is less error-prone (in
case the list is checked between list_add and a fail).

But what if we put it at the end?
What is the chance of a kzalloc of 4 pointers (struct direct_window)
failing after walk_system_ram_range?

Is it not worthy doing that for making enable_ddw() easier to
understand?

Best regards,
Leonardo