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authorDan Williams <dan.j.williams@intel.com>2019-07-19 01:58:29 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2019-07-19 03:08:07 +0300
commita0653406a3a671c1609d54835f0443869525ca30 (patch)
tree22ddbbe3289e910dca80afe988901ac143a707a4
parentba72b4c8cf60e452cf6f0258ed9ee697957b7dfd (diff)
downloadlinux-a0653406a3a671c1609d54835f0443869525ca30.tar.xz
mm: document ZONE_DEVICE memory-model implications
Explain the general mechanisms of 'ZONE_DEVICE' pages and list the users of 'devm_memremap_pages()'. [dan.j.williams@intel.com: update ZONE_DEVICE memory model documentation] Link: http://lkml.kernel.org/r/156109575458.1409767.1885676287099277666.stgit@dwillia2-desk3.amr.corp.intel.com Link: http://lkml.kernel.org/r/156092354985.979959.15763234410543451710.stgit@dwillia2-desk3.amr.corp.intel.com Signed-off-by: Dan Williams <dan.j.williams@intel.com> Reported-by: Mike Rapoport <rppt@linux.ibm.com> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com> Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> [ppc64] Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Hildenbrand <david@redhat.com> Cc: Jane Chu <jane.chu@oracle.com> Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Jérôme Glisse <jglisse@redhat.com> Cc: Logan Gunthorpe <logang@deltatee.com> Cc: Michal Hocko <mhocko@suse.com> Cc: Oscar Salvador <osalvador@suse.de> Cc: Pavel Tatashin <pasha.tatashin@soleen.com> Cc: Toshi Kani <toshi.kani@hpe.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Wei Yang <richardw.yang@linux.intel.com> Cc: Jason Gunthorpe <jgg@mellanox.com> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--Documentation/vm/memory-model.rst40
1 files changed, 40 insertions, 0 deletions
diff --git a/Documentation/vm/memory-model.rst b/Documentation/vm/memory-model.rst
index 382f72ace1fc..58a12376b7df 100644
--- a/Documentation/vm/memory-model.rst
+++ b/Documentation/vm/memory-model.rst
@@ -181,3 +181,43 @@ that is eventually passed to vmemmap_populate() through a long chain
of function calls. The vmemmap_populate() implementation may use the
`vmem_altmap` along with :c:func:`altmap_alloc_block_buf` helper to
allocate memory map on the persistent memory device.
+
+ZONE_DEVICE
+===========
+The `ZONE_DEVICE` facility builds upon `SPARSEMEM_VMEMMAP` to offer
+`struct page` `mem_map` services for device driver identified physical
+address ranges. The "device" aspect of `ZONE_DEVICE` relates to the fact
+that the page objects for these address ranges are never marked online,
+and that a reference must be taken against the device, not just the page
+to keep the memory pinned for active use. `ZONE_DEVICE`, via
+:c:func:`devm_memremap_pages`, performs just enough memory hotplug to
+turn on :c:func:`pfn_to_page`, :c:func:`page_to_pfn`, and
+:c:func:`get_user_pages` service for the given range of pfns. Since the
+page reference count never drops below 1 the page is never tracked as
+free memory and the page's `struct list_head lru` space is repurposed
+for back referencing to the host device / driver that mapped the memory.
+
+While `SPARSEMEM` presents memory as a collection of sections,
+optionally collected into memory blocks, `ZONE_DEVICE` users have a need
+for smaller granularity of populating the `mem_map`. Given that
+`ZONE_DEVICE` memory is never marked online it is subsequently never
+subject to its memory ranges being exposed through the sysfs memory
+hotplug api on memory block boundaries. The implementation relies on
+this lack of user-api constraint to allow sub-section sized memory
+ranges to be specified to :c:func:`arch_add_memory`, the top-half of
+memory hotplug. Sub-section support allows for 2MB as the cross-arch
+common alignment granularity for :c:func:`devm_memremap_pages`.
+
+The users of `ZONE_DEVICE` are:
+
+* pmem: Map platform persistent memory to be used as a direct-I/O target
+ via DAX mappings.
+
+* hmm: Extend `ZONE_DEVICE` with `->page_fault()` and `->page_free()`
+ event callbacks to allow a device-driver to coordinate memory management
+ events related to device-memory, typically GPU memory. See
+ Documentation/vm/hmm.rst.
+
+* p2pdma: Create `struct page` objects to allow peer devices in a
+ PCI/-E topology to coordinate direct-DMA operations between themselves,
+ i.e. bypass host memory.