On Tuesday, 30 March 2021 8:13:32 PM AEDT David Hildenbrand wrote: > External email: Use caution opening links or attachments > > > On 29.03.21 03:37, Alistair Popple wrote: > > On Friday, 26 March 2021 7:57:51 PM AEDT David Hildenbrand wrote: > >> On 26.03.21 02:20, Alistair Popple wrote: > >>> request_free_mem_region() is used to find an empty range of physical > >>> addresses for hotplugging ZONE_DEVICE memory. It does this by iterating > >>> over the range of possible addresses using region_intersects() to see if > >>> the range is free. > >> > >> Just a high-level question: how does this iteract with memory > >> hot(un)plug? IOW, how defines and manages the "range of possible > >> addresses" ? > > > > Both the driver and the maximum physical address bits available define the > > range of possible addresses for device private memory. From > > __request_free_mem_region(): > > > > end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1); > > addr = end - size + 1UL; > > > > There is no lower address range bound here so it is effectively zero. The code > > will try to allocate the highest possible physical address first and continue > > searching down for a free block. Does that answer your question? > > Oh, sorry, the fist time I had a look I got it wrong - I thought (1UL << > MAX_PHYSMEM_BITS) would be the lower address limit. That looks indeed > problematic to me. > > You might end up reserving an iomem region that could be used e.g., by > memory hotplug code later. If someone plugs a DIMM or adds memory via > different approaches (virtio-mem), memory hotplug (via add_memory()) > would fail. > > You never should be touching physical memory area reserved for memory > hotplug, i.e., via SRAT. > > What is the expectation here? Most drivers call request_free_mem_region() with iomem_resource as the base. So zone device private pages currently tend to get allocated from the top of that. By definition ZONE_DEVICE private pages are unaddressable from the CPU. So in terms of expectation I think all that is really required for ZONE_DEVICE private pages (at least for Nouveau) is a valid range of physical addresses that allow page_to_pfn() and pfn_to_page() to work correctly. To make this work drivers add the pages via memremap_pages() -> pagemap_range() -> add_pages(). - Alistair > -- > Thanks, > > David / dhildenb >
