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

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Date:   Tue, 11 Jul 2017 10:57:44 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Balbir Singh <bsingharora@...il.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, 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.

[...]

> > 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.

[...]

> > @@ -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.

> 
> > +		else if (is_device_public_page(page)) {
> > +			entry = pte_mkold(mk_pte(page, READ_ONCE(vma->vm_page_prot)));
> > +			if (vma->vm_flags & VM_WRITE)
> > +				entry = pte_mkwrite(pte_mkdirty(entry));
> > +			entry = pte_mkdevmap(entry);
> > +		}
> > +#endif /* IS_ENABLED(CONFIG_DEVICE_PUBLIC) */