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    Subject[PATCH 6/9] mm/memory-hotplug: allow memory resources to be children
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    The mm/resource.c code is used to manage the physical address
    space. We can view the current resource configuration in
    /proc/iomem. An example of this is at the bottom of this
    description.

    The nvdimm subsystem "owns" the physical address resources which
    map to persistent memory and has resources inserted for them as
    "Persistent Memory". We want to use this persistent memory, but
    as volatile memory, just like RAM. The best way to do this is
    to leave the existing resource in place, but add a "System RAM"
    resource underneath it. This clearly communicates the ownership
    relationship of this memory.

    The request_resource_conflict() API only deals with the
    top-level resources. Replace it with __request_region() which
    will search for !IORESOURCE_BUSY areas lower in the resource
    tree than the top level.

    We also rework the old error message a bit since we do not get
    the conflicting entry back: only an indication that we *had* a
    conflict.

    We *could* also simply truncate the existing top-level
    "Persistent Memory" resource and take over the released address
    space. But, this means that if we ever decide to hot-unplug the
    "RAM" and give it back, we need to recreate the original setup,
    which may mean going back to the BIOS tables.

    This should have no real effect on the existing collision
    detection because the areas that truly conflict should be marked
    IORESOURCE_BUSY.

    00000000-00000fff : Reserved
    00001000-0009fbff : System RAM
    0009fc00-0009ffff : Reserved
    000a0000-000bffff : PCI Bus 0000:00
    000c0000-000c97ff : Video ROM
    000c9800-000ca5ff : Adapter ROM
    000f0000-000fffff : Reserved
    000f0000-000fffff : System ROM
    00100000-9fffffff : System RAM
    01000000-01e071d0 : Kernel code
    01e071d1-027dfdff : Kernel data
    02dc6000-0305dfff : Kernel bss
    a0000000-afffffff : Persistent Memory (legacy)
    a0000000-a7ffffff : System RAM
    b0000000-bffdffff : System RAM
    bffe0000-bfffffff : Reserved
    c0000000-febfffff : PCI Bus 0000:00

    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc: Dave Jiang <dave.jiang@intel.com>
    Cc: Ross Zwisler <zwisler@kernel.org>
    Cc: Vishal Verma <vishal.l.verma@intel.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: linux-nvdimm@lists.01.org
    Cc: linux-kernel@vger.kernel.org
    Cc: linux-mm@kvack.org
    Cc: Huang Ying <ying.huang@intel.com>
    Cc: Fengguang Wu <fengguang.wu@intel.com>

    ---

    b/mm/memory_hotplug.c | 31 ++++++++++++++-----------------
    1 file changed, 14 insertions(+), 17 deletions(-)

    diff -puN mm/memory_hotplug.c~mm-memory-hotplug-allow-memory-resource-to-be-child mm/memory_hotplug.c
    --- a/mm/memory_hotplug.c~mm-memory-hotplug-allow-memory-resource-to-be-child 2018-10-22 13:12:23.570930388 -0700
    +++ b/mm/memory_hotplug.c 2018-10-22 13:12:23.573930388 -0700
    @@ -99,24 +99,21 @@ void mem_hotplug_done(void)
    /* add this memory to iomem resource */
    static struct resource *register_memory_resource(u64 start, u64 size)
    {
    - struct resource *res, *conflict;
    - res = kzalloc(sizeof(struct resource), GFP_KERNEL);
    - if (!res)
    - return ERR_PTR(-ENOMEM);
    + struct resource *res;
    + unsigned long flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
    + char resource_name[] = "System RAM";

    - res->name = "System RAM";
    - res->start = start;
    - res->end = start + size - 1;
    - res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
    - conflict = request_resource_conflict(&iomem_resource, res);
    - if (conflict) {
    - if (conflict->desc == IORES_DESC_DEVICE_PRIVATE_MEMORY) {
    - pr_debug("Device unaddressable memory block "
    - "memory hotplug at %#010llx !\n",
    - (unsigned long long)start);
    - }
    - pr_debug("System RAM resource %pR cannot be added\n", res);
    - kfree(res);
    + /*
    + * Request ownership of the new memory range. This might be
    + * a child of an existing resource that was present but
    + * not marked as busy.
    + */
    + res = __request_region(&iomem_resource, start, size,
    + resource_name, flags);
    +
    + if (!res) {
    + pr_debug("Unable to reserve System RAM region: %016llx->%016llx\n",
    + start, start + size);
    return ERR_PTR(-EEXIST);
    }
    return res;
    _
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