linux-kernel - Re: [PATCH v1 06/10] powerpc/pseries/iommu: Add ddw_list

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Message-ID: <f80040bf941755469918fb75cf520590a4a5e3db.camel@gmail.com>
Date:   Thu, 27 Aug 2020 19:11:28 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Joel Stanley <joel@....id.au>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Brian King <brking@...ux.vnet.ibm.com>,
        Murilo Fossa Vicentini <muvic@...ux.ibm.com>,
        David Dai <zdai@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 06/10] powerpc/pseries/iommu: Add ddw_list_add()
 helper

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