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

[Leonardo Bras <leobras.c@gmail.com>]

On Fri, 2020-08-28 at 11:58 +1000, Alexey Kardashevskiy wrote:
> 
> On 28/08/2020 08:11, Leonardo Bras wrote:
> > 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).
> 
> Performance was not in my mind at all.
> 
> I noticed you remove from a list with a lock help and it was not there
> before and there is a bunch on labels on the exit path and started
> looking for list_add() and if you do not double remove from the list.
> 
> 
> > 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?
> 
> This is not about chances really, it is about readability. If let's say
> kmalloc failed, you just to the error exit label and simply call kfree()
> on that pointer, kfree will do nothing if it is NULL already, simple.
> list_del() does not have this simplicity.
> 
> 
> > Is it not worthy doing that for making enable_ddw() easier to
> > understand?
> 
> This is my goal here :)

Ok, it makes sense to me now. 
I tried creating list_add() to keep everything related to list-adding
into a single place, instead of splitting it around the other stuff,
but now I understand that the code may look more complex than it was
before, because of the failing path increasing in size.

For me it was strange creating a list entry end not list_add()ing it
right away, but maybe it's something worth to get used to, as it may
increase the failing path simplicity, since list_add() don't fail.

I will try to see if the ddw_list_add() routine would become a useful
ddw_list_entry(), but if not, I will remove this patch.

Alexey, Thank you for reviewing this series!
Best regards,

Leonardo