On Tue, Jun 18, 2019 at 10:52:29PM -0700, Dan Williams wrote: > Explain the general mechanisms of 'ZONE_DEVICE' pages and list the users > of 'devm_memremap_pages()'. > > Cc: Jonathan Corbet <corbet@xxxxxxx> > Reported-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> With one nit below Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> > --- > Documentation/vm/memory-model.rst | 39 +++++++++++++++++++++++++++++++++++++ > 1 file changed, 39 insertions(+) > > diff --git a/Documentation/vm/memory-model.rst b/Documentation/vm/memory-model.rst > index 382f72ace1fc..e0af47e02e78 100644 > --- a/Documentation/vm/memory-model.rst > +++ b/Documentation/vm/memory-model.rst > @@ -181,3 +181,42 @@ 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 `PMD_SIZE` as the minimum > +alignment granularity for :c:func:`devm_memremap_pages`. > + > +The users of `ZONE_DEVICE` are: Sphinx wants an empty line here: /home/rapoport/git/linux-docs/Documentation/vm/memory-model.rst:213: ERROR: Unexpected indentation. > +* 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. > -- Sincerely yours, Mike.
