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author | Dan Williams <dan.j.williams@intel.com> | 2019-07-18 15:58:29 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-07-18 17:08:07 -0700 |
commit | a0653406a3a671c1609d54835f0443869525ca30 (patch) | |
tree | 22ddbbe3289e910dca80afe988901ac143a707a4 /Documentation/vm | |
parent | ba72b4c8cf60e452cf6f0258ed9ee697957b7dfd (diff) | |
download | lwn-a0653406a3a671c1609d54835f0443869525ca30.tar.gz lwn-a0653406a3a671c1609d54835f0443869525ca30.zip |
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>
Diffstat (limited to 'Documentation/vm')
-rw-r--r-- | Documentation/vm/memory-model.rst | 40 |
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. |