linux-kernel - Re: [PATCH 2/5] mm/device-public-memory: device memory cache coherent with CPU v2

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Date:   Wed, 12 Jul 2017 15:50:15 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Balbir Singh <balbirs@....ibm.com>,
        Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 2/5] mm/device-public-memory: device memory cache
 coherent with CPU v2

On Tue, 11 Jul 2017 10:57:44 -0400
Jerome Glisse <jglisse@...hat.com> wrote:

> On Tue, Jul 11, 2017 at 02:12:15PM +1000, Balbir Singh wrote:
> > On Mon,  3 Jul 2017 17:14:12 -0400
> > Jérôme Glisse <jglisse@...hat.com> wrote:
> >   
> > > Platform with advance system bus (like CAPI or CCIX) allow device
> > > memory to be accessible from CPU in a cache coherent fashion. Add
> > > a new type of ZONE_DEVICE to represent such memory. The use case
> > > are the same as for the un-addressable device memory but without
> > > all the corners cases.
> > >  
> > 
> > Looks good overall, some comments inline.
> >    
> 
> [...]
> 
> > >  /*
> > > @@ -92,6 +100,8 @@ enum memory_type {
> > >   * The page_free() callback is called once the page refcount reaches 1
> > >   * (ZONE_DEVICE pages never reach 0 refcount unless there is a refcount bug.
> > >   * This allows the device driver to implement its own memory management.)
> > > + *
> > > + * For MEMORY_DEVICE_CACHE_COHERENT only the page_free() callback matter.  
> > 
> > Correct, but I wonder if we should in the long term allow for minor faults
> > (due to coherency) via this interface?  
> 
> This is something we can explore latter on.
> 
> [...]
> 

Agreed

> > > diff --git a/kernel/memremap.c b/kernel/memremap.c
> > > index e82456c39a6a..da74775f2247 100644
> > > --- a/kernel/memremap.c
> > > +++ b/kernel/memremap.c
> > > @@ -466,7 +466,7 @@ struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
> > >  
> > >  
> > >  #ifdef CONFIG_DEVICE_PRIVATE  
> > 
> > Does the #ifdef above need to go as well?  
> 
> Good catch i should make that conditional on DEVICE_PUBLIC or whatever
> the name endup to be. I will make sure i test without DEVICE_PRIVATE
> config before posting again.
> 
> [...]
> 

I've been testing with this off, I should have sent you a patch, but
I thought I'd also update in the review.

> > > @@ -2541,11 +2551,21 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
> > >  	 */
> > >  	__SetPageUptodate(page);
> > >  
> > > -	if (is_zone_device_page(page) && is_device_private_page(page)) {
> > > -		swp_entry_t swp_entry;
> > > +	if (is_zone_device_page(page)) {
> > > +		if (is_device_private_page(page)) {
> > > +			swp_entry_t swp_entry;
> > >  
> > > -		swp_entry = make_device_private_entry(page, vma->vm_flags & VM_WRITE);
> > > -		entry = swp_entry_to_pte(swp_entry);
> > > +			swp_entry = make_device_private_entry(page, vma->vm_flags & VM_WRITE);
> > > +			entry = swp_entry_to_pte(swp_entry);
> > > +		}
> > > +#if IS_ENABLED(CONFIG_DEVICE_PUBLIC)  
> > 
> > Do we need this #if check? is_device_public_page(page)
> > will return false if the config is disabled  
> 
> pte_mkdevmap() is not define if ZONE_DEVICE is not enabled hence
> i had to protect this with #if/#endif to avoid build error.

pte_mkdevmap is always defined, could you please share the build
error.


Balbir Singh.