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Date:   Mon, 16 Jan 2017 15:13:12 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM v16 04/15] mm/ZONE_DEVICE/unaddressable: add support for
 un-addressable device memory v2

On Mon, Jan 16, 2017 at 11:31:39AM -0800, Dan Williams wrote:
> On Mon, Jan 16, 2017 at 7:17 AM, Jerome Glisse <jglisse@...hat.com> wrote:
> > On Sun, Jan 15, 2017 at 11:05:43PM -0800, Dan Williams wrote:
> >> On Thu, Jan 12, 2017 at 8:30 AM, Jérôme Glisse <jglisse@...hat.com> wrote:
> >> > This add support for un-addressable device memory. Such memory is hotpluged
> >> > only so we can have struct page but we should never map them as such memory
> >> > can not be accessed by CPU. For that reason it uses a special swap entry for
> >> > CPU page table entry.
> >> >
> >> > This patch implement all the logic from special swap type to handling CPU
> >> > page fault through a callback specified in the ZONE_DEVICE pgmap struct.
> >> >
> >> > Architecture that wish to support un-addressable device memory should make
> >> > sure to never populate the kernel linar mapping for the physical range.
> >> >
> >> > This feature potentially breaks memory hotplug unless every driver using it
> >> > magically predicts the future addresses of where memory will be hotplugged.
> >> >
> >> > Changed since v1:
> >> >   - Add unaddressable memory resource descriptor enum
> >> >   - Explain why memory hotplug can fail because of un-addressable memory
> >>
> >> How can we merge this with a known potential regression?
> >
> > This only affect people using HMM it does not affect people that do not
> > use it. Like i said for time being people that want HMM are ok with that.
> > I haven't look yet on how to allow struct page about MAX_PHYSMEM_BITS
> >
> >>
> >> >
> >> > Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> >> > Cc: Dan Williams <dan.j.williams@...el.com>
> >> > Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> >> > ---
> >> >  drivers/dax/pmem.c                |  4 +--
> >> >  drivers/nvdimm/pmem.c             |  6 ++--
> >> >  fs/proc/task_mmu.c                | 10 +++++-
> >> >  include/linux/ioport.h            |  1 +
> >> >  include/linux/memory_hotplug.h    |  7 ++++
> >> >  include/linux/memremap.h          | 29 +++++++++++++++--
> >> >  include/linux/swap.h              | 18 +++++++++--
> >> >  include/linux/swapops.h           | 67 +++++++++++++++++++++++++++++++++++++++
> >> >  kernel/memremap.c                 | 43 +++++++++++++++++++++++--
> >> >  mm/Kconfig                        | 12 +++++++
> >> >  mm/memory.c                       | 64 ++++++++++++++++++++++++++++++++++++-
> >> >  mm/memory_hotplug.c               | 10 ++++--
> >> >  mm/mprotect.c                     | 12 +++++++
> >> >  tools/testing/nvdimm/test/iomap.c |  3 +-
> >> >  14 files changed, 269 insertions(+), 17 deletions(-)
> >> >
> >> > diff --git a/drivers/dax/pmem.c b/drivers/dax/pmem.c
> >> > index 66af7b1..c50b58d 100644
> >> > --- a/drivers/dax/pmem.c
> >> > +++ b/drivers/dax/pmem.c
> >> > @@ -111,8 +111,8 @@ static int dax_pmem_probe(struct device *dev)
> >> >         if (rc)
> >> >                 return rc;
> >> >
> >> > -       addr = devm_memremap_pages(dev, &res, &dax_pmem->ref,
> >> > -                                  altmap, NULL, NULL);
> >> > +       addr = devm_memremap_pages(dev, &res, &dax_pmem->ref, altmap,
> >> > +                                  NULL, NULL, NULL, NULL, MEMORY_DEVICE);
> >> >         if (IS_ERR(addr))
> >> >                 return PTR_ERR(addr);
> >> >
> >> > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> >> > index f2f1904..8166a56 100644
> >> > --- a/drivers/nvdimm/pmem.c
> >> > +++ b/drivers/nvdimm/pmem.c
> >> > @@ -282,7 +282,8 @@ static int pmem_attach_disk(struct device *dev,
> >> >         pmem->pfn_flags = PFN_DEV;
> >> >         if (is_nd_pfn(dev)) {
> >> >                 addr = devm_memremap_pages(dev, &pfn_res, &q->q_usage_counter,
> >> > -                                          altmap, NULL, NULL);
> >> > +                                          altmap, NULL, NULL, NULL,
> >> > +                                          NULL, MEMORY_DEVICE);
> >> >                 pfn_sb = nd_pfn->pfn_sb;
> >> >                 pmem->data_offset = le64_to_cpu(pfn_sb->dataoff);
> >> >                 pmem->pfn_pad = resource_size(res) - resource_size(&pfn_res);
> >> > @@ -292,7 +293,8 @@ static int pmem_attach_disk(struct device *dev,
> >> >         } else if (pmem_should_map_pages(dev)) {
> >> >                 addr = devm_memremap_pages(dev, &nsio->res,
> >> >                                            &q->q_usage_counter,
> >> > -                                          NULL, NULL, NULL);
> >> > +                                          NULL, NULL, NULL, NULL,
> >> > +                                          NULL, MEMORY_DEVICE);
> >> >                 pmem->pfn_flags |= PFN_MAP;
> >> >         } else
> >> >                 addr = devm_memremap(dev, pmem->phys_addr,
> >> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> >> > index 958f325..9a6ab71 100644
> >> > --- a/fs/proc/task_mmu.c
> >> > +++ b/fs/proc/task_mmu.c
> >> > @@ -535,8 +535,11 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
> >> >                         } else {
> >> >                                 mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
> >> >                         }
> >> > -               } else if (is_migration_entry(swpent))
> >> > +               } else if (is_migration_entry(swpent)) {
> >> >                         page = migration_entry_to_page(swpent);
> >> > +               } else if (is_device_entry(swpent)) {
> >> > +                       page = device_entry_to_page(swpent);
> >> > +               }
> >> >         } else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap
> >> >                                                         && pte_none(*pte))) {
> >> >                 page = find_get_entry(vma->vm_file->f_mapping,
> >> > @@ -699,6 +702,8 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
> >> >
> >> >                 if (is_migration_entry(swpent))
> >> >                         page = migration_entry_to_page(swpent);
> >> > +               if (is_device_entry(swpent))
> >> > +                       page = device_entry_to_page(swpent);
> >> >         }
> >> >         if (page) {
> >> >                 int mapcount = page_mapcount(page);
> >> > @@ -1182,6 +1187,9 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> >> >                 flags |= PM_SWAP;
> >> >                 if (is_migration_entry(entry))
> >> >                         page = migration_entry_to_page(entry);
> >> > +
> >> > +               if (is_device_entry(entry))
> >> > +                       page = device_entry_to_page(entry);
> >> >         }
> >> >
> >> >         if (page && !PageAnon(page))
> >> > diff --git a/include/linux/ioport.h b/include/linux/ioport.h
> >> > index 6230064..d154a18 100644
> >> > --- a/include/linux/ioport.h
> >> > +++ b/include/linux/ioport.h
> >> > @@ -130,6 +130,7 @@ enum {
> >> >         IORES_DESC_ACPI_NV_STORAGE              = 3,
> >> >         IORES_DESC_PERSISTENT_MEMORY            = 4,
> >> >         IORES_DESC_PERSISTENT_MEMORY_LEGACY     = 5,
> >> > +       IORES_DESC_UNADDRESSABLE_MEMORY         = 6,
> >> >  };
> >> >
> >> >  /* helpers to define resources */
> >> > diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> >> > index 3f50eb8..e7c5dc6 100644
> >> > --- a/include/linux/memory_hotplug.h
> >> > +++ b/include/linux/memory_hotplug.h
> >> > @@ -285,15 +285,22 @@ extern int zone_for_memory(int nid, u64 start, u64 size, int zone_default,
> >> >   * never relied on struct page migration so far and new user of might also
> >> >   * prefer avoiding struct page migration.
> >> >   *
> >> > + * For device memory (which use ZONE_DEVICE) we want differentiate between CPU
> >> > + * accessible memory (persitent memory, device memory on an architecture with a
> >> > + * system bus that allow transparent access to device memory) and unaddressable
> >> > + * memory (device memory that can not be accessed by CPU directly).
> >> > + *
> >> >   * New non device memory specific flags can be added if ever needed.
> >> >   *
> >> >   * MEMORY_REGULAR: regular system memory
> >> >   * DEVICE_MEMORY: device memory create a ZONE_DEVICE zone for it
> >> >   * DEVICE_MEMORY_ALLOW_MIGRATE: page in that device memory ca be migrated
> >> > + * MEMORY_DEVICE_UNADDRESSABLE: un-addressable memory (CPU can not access it)
> >> >   */
> >> >  #define MEMORY_NORMAL 0
> >> >  #define MEMORY_DEVICE (1 << 0)
> >> >  #define MEMORY_DEVICE_ALLOW_MIGRATE (1 << 1)
> >> > +#define MEMORY_DEVICE_UNADDRESSABLE (1 << 2)
> >> >
> >> >  extern int arch_add_memory(int nid, u64 start, u64 size, int flags);
> >> >  extern int offline_pages(unsigned long start_pfn, unsigned long nr_pages);
> >> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> >> > index 582561f..4b9f02c 100644
> >> > --- a/include/linux/memremap.h
> >> > +++ b/include/linux/memremap.h
> >> > @@ -35,31 +35,42 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
> >> >  }
> >> >  #endif
> >> >
> >> > +typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
> >> > +                               unsigned long addr,
> >> > +                               struct page *page,
> >> > +                               unsigned flags,
> >> > +                               pmd_t *pmdp);
> >> >  typedef void (*dev_page_free_t)(struct page *page, void *data);
> >> >
> >> >  /**
> >> >   * struct dev_pagemap - metadata for ZONE_DEVICE mappings
> >> > + * @page_fault: callback when CPU fault on an un-addressable device page
> >> >   * @page_free: free page callback when page refcount reach 1
> >> >   * @altmap: pre-allocated/reserved memory for vmemmap allocations
> >> >   * @res: physical address range covered by @ref
> >> >   * @ref: reference count that pins the devm_memremap_pages() mapping
> >> >   * @dev: host device of the mapping for debug
> >> >   * @data: privata data pointer for page_free
> >> > + * @flags: device memory flags (look for MEMORY_DEVICE_* memory_hotplug.h)
> >> >   */
> >> >  struct dev_pagemap {
> >> > +       dev_page_fault_t page_fault;
> >> >         dev_page_free_t page_free;
> >> >         struct vmem_altmap *altmap;
> >> >         const struct resource *res;
> >> >         struct percpu_ref *ref;
> >> >         struct device *dev;
> >> >         void *data;
> >> > +       int flags;
> >> >  };
> >>
> >> dev_pagemap is only meant for get_user_pages() to do lookups of ptes
> >> with _PAGE_DEVMAP and take a reference against the hosting device..
> >
> > And i want to build on top of that to extend _PAGE_DEVMAP to support
> > a new usecase for unaddressable device memory.
> >
> >>
> >> Why can't HMM use the typical vm_operations_struct fault path and push
> >> more of these details to a driver rather than the core?
> >
> > Because the vm_operations_struct has nothing to do with the device.
> > We are talking about regular vma here. Think malloc, mmap, share
> > memory, ...  not about mmap(/dev/thedevice,...)
> >
> > So the vm_operations_struct is never under device control and we can
> > not, nor want to, rely on that.
> 
> Can you explain more what's behind that "can not, nor want to"
> statement? It seems to me that any awkwardness of moving to a
> standalone device file interface is less than a maintaining a new /
> parallel mm fault path through dev_pagemap.

The whole point of HMM is to allow transparent usage of process address
space on to a device like GPU. So it imply any vma (vm_area_struct) that
result from usual mmap (ie any mmap either PRIVATE or SHARE as long as it
is not a an mmap of a device file).

It means that application can use malloc or the usual memory allocation
primitive of the langage (c++, rust, python, ...) and directly use the
memory it gets from that with the device.

Device like GPU have a large pool of device memory that is not accessible
by the CPU. This device memory has 10 times more bandwidth than system
memory and has better latency then PCIE. Hence for the whole thing to
make sense you need to allow to use it.

For that you need to allow migration from system memory to device memory.
Because you can not rely on special userspace allocator you have to
assume that the vma (vm_area_struct) is a regular one. So we are left
with having struct page for the device memory to allow migration to
work without requiring too much changes to existing mm.

Because device memory is not accessible by the CPU, you can not allow
anyone to pin it and thus get_user_page* must trigger a migration back
as CPU page fault would.


> > So what we looking for here is struct page that can behave mostly
> > like anyother except that we do not want to allow GUP to take a
> > reference almost exactly what ZONE_DEVICE already provide.
> >
> > So do you have any fundamental objections to this patchset ? And if
> > so, how do you propose i solve the problem i am trying to address ?
> > Because hardware exist today and without something like HMM we will
> > not be able to support such hardware.
> 
> My pushback stems from it being a completely different use case for
> devm_memremap_pages(), as evidenced by it growing from 4 arguments to
> 9, and the ongoing maintenance overhead of understanding HMM
> requirements when updating the pmem usage of ZONE_DEVICE.

I rather reuse something existing and modify it to support more use case
than try to add ZONE_DEVICE2 or ZONE_DEVICE_I_AM_DIFFERENT. I have made
sure that my modifications to ZONE_DEVICE can be use without HMM. It is
just a generic interface to support page fault and to allow to track last
user of a device page. Both can be use indepentently from each other.

To me the whole point of kernel is trying to share infrastructure accross
as many hardware as possible and i am doing just that. I do not think HMM
should be block because something that use to be for one specific use case
now support 2 use cases. I am not breaking anything existing. Is it more
work for you ? Maybe, but at Red Hat we intend to support it for as long
as it is needed so you always have some one to talk to if you want to
update ZONE_DEVICE.

Cheers,
Jérôme

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