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

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Date:   Wed, 5 Jul 2017 09:15:35 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.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 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.