linux-kernel - Re: [PATCH 1/5] mm/persistent-memory: match IORES_DESC name and enum memory

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Message-ID: <20170705184933.GD3305@redhat.com>
Date:   Wed, 5 Jul 2017 14:49:33 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "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>,
        Balbir Singh <bsingharora@...il.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, 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