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Date:   Tue, 11 Jul 2017 13:48:21 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 1/5] mm/persistent-memory: match IORES_DESC name and
 enum memory_type one

On Wed, 5 Jul 2017 14:49:33 -0400
Jerome Glisse <jglisse@...hat.com> wrote:

> On Wed, Jul 05, 2017 at 09:15:35AM -0700, Dan Williams wrote:
> > On Wed, Jul 5, 2017 at 7:25 AM, Jerome Glisse <jglisse@...hat.com> wrote:  
> > > On Mon, Jul 03, 2017 at 04:49:18PM -0700, Dan Williams wrote:  
> > >> On Mon, Jul 3, 2017 at 2:14 PM, Jérôme Glisse <jglisse@...hat.com> wrote:  
> > >> > Use consistent name between IORES_DESC and enum memory_type, rename
> > >> > MEMORY_DEVICE_PUBLIC to MEMORY_DEVICE_PERSISTENT. This is to free up
> > >> > the public name for CDM (cache coherent device memory) for which the
> > >> > term public is a better match.
> > >> >
> > >> > Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> > >> > Cc: Dan Williams <dan.j.williams@...el.com>
> > >> > Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > >> > ---
> > >> >  include/linux/memremap.h | 4 ++--
> > >> >  kernel/memremap.c        | 2 +-
> > >> >  2 files changed, 3 insertions(+), 3 deletions(-)
> > >> >
> > >> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> > >> > index 57546a07a558..2299cc2d387d 100644
> > >> > --- a/include/linux/memremap.h
> > >> > +++ b/include/linux/memremap.h
> > >> > @@ -41,7 +41,7 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
> > >> >   * Specialize ZONE_DEVICE memory into multiple types each having differents
> > >> >   * usage.
> > >> >   *
> > >> > - * MEMORY_DEVICE_PUBLIC:
> > >> > + * MEMORY_DEVICE_PERSISTENT:
> > >> >   * Persistent device memory (pmem): struct page might be allocated in different
> > >> >   * memory and architecture might want to perform special actions. It is similar
> > >> >   * to regular memory, in that the CPU can access it transparently. However,
> > >> > @@ -59,7 +59,7 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
> > >> >   * include/linux/hmm.h and Documentation/vm/hmm.txt.
> > >> >   */
> > >> >  enum memory_type {
> > >> > -       MEMORY_DEVICE_PUBLIC = 0,
> > >> > +       MEMORY_DEVICE_PERSISTENT = 0,
> > >> >         MEMORY_DEVICE_PRIVATE,
> > >> >  };
> > >> >
> > >> > diff --git a/kernel/memremap.c b/kernel/memremap.c
> > >> > index b9baa6c07918..e82456c39a6a 100644
> > >> > --- a/kernel/memremap.c
> > >> > +++ b/kernel/memremap.c
> > >> > @@ -350,7 +350,7 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
> > >> >         }
> > >> >         pgmap->ref = ref;
> > >> >         pgmap->res = &page_map->res;
> > >> > -       pgmap->type = MEMORY_DEVICE_PUBLIC;
> > >> > +       pgmap->type = MEMORY_DEVICE_PERSISTENT;
> > >> >         pgmap->page_fault = NULL;
> > >> >         pgmap->page_free = NULL;
> > >> >         pgmap->data = NULL;  
> > >>
> > >> I think we need a different name. There's nothing "persistent" about
> > >> the devm_memremap_pages() path. Why can't they share name, is the only
> > >> difference coherence? I'm thinking something like:
> > >>
> > >> MEMORY_DEVICE_PRIVATE
> > >> MEMORY_DEVICE_COHERENT /* persistent memory and coherent devices */
> > >> MEMORY_DEVICE_IO /* "public", but not coherent */  
> > >
> > > No that would not work. Device public (in the context of this patchset)
> > > is like device private ie device public page can be anywhere inside a
> > > process address space either as anonymous memory page or as file back
> > > page of regular filesystem (ie vma->ops is not pointing to anything
> > > specific to the device memory).
> > >
> > > As such device public is different from how persistent memory is use
> > > and those the cache coherency being the same between the two kind of
> > > memory is not a discerning factor. So i need to distinguish between
> > > persistent memory and device public memory.
> > >
> > > I believe keeping enum memory_type close to IORES_DESC naming is the
> > > cleanest way to do that but i am open to other name suggestion.
> > >  
> > 
> > The IORES_DESC has nothing to do with how the memory range is handled
> > by the core mm. It sounds like the distinction this is trying to make
> > is between MEMORY_DEVICE_{PUBLIC,PRIVATE} and MEMORY_DEVICE_HOST.
> > Where a "host" memory range is one that does not need coordination
> > with a specific device.  
> 
> I want to distinguish between:
>   - device memory that is not accessible by the CPU
>   - device memory that is accessible by the CPU just like regular
>     memory
>   - existing user of devm_memremap_pages() which is persistent memory
>     (only pmem seems to call devm_memremap_pages()) that is use like a
>     filesystem or block device and thus isn't use like generic page in
>     a process address space
> 
> So if existing user of devm_memremap_pages() are only persistent memory
> then it made sense to match the IORES_DESC we are expecting to see on
> see such memory.
> 
> For public device memory (in the sense introduced by this patchset) i
> do not know how it will be described by IORES_DESC. i think first folks
> with it are IBM with CAPI and i am not sure they defined something for
> that already.
> 
> I am open to any name beside public (well any reasonable name :)) but
> i do need to be able to distinguish persistent memory as use today from
> this device memory.
> 
> Cheers,
> Jérôme

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