linux-kernel - Re: [PATCH 3/3] dax/kmem: Always enroll hotplugged memory for memmap_on

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Date:   Thu, 13 Jul 2023 17:23:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "aneesh.kumar@...ux.ibm.com" <aneesh.kumar@...ux.ibm.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "lenb@...nel.org" <lenb@...nel.org>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 3/3] dax/kmem: Always enroll hotplugged memory for
 memmap_on_memory

On 13.07.23 17:15, Verma, Vishal L wrote:
> On Thu, 2023-07-13 at 09:23 +0200, David Hildenbrand wrote:
>> On 13.07.23 08:45, Verma, Vishal L wrote:
>>>
>>> I'm taking a shot at implementing the splitting internally in
>>> memory_hotplug.c. The caller (kmem) side does become trivial with this
>>> approach, but there's a slight complication if I don't have the module
>>> param override (patch 1 of this series).
>>>
>>> The kmem diff now looks like:
>>>
>>>      diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
>>>      index 898ca9505754..8be932f63f90 100644
>>>      --- a/drivers/dax/kmem.c
>>>      +++ b/drivers/dax/kmem.c
>>>      @@ -105,6 +105,8 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>>              data->mgid = rc;
>>>       
>>>              for (i = 0; i < dev_dax->nr_range; i++) {
>>>      +               mhp_t mhp_flags = MHP_NID_IS_MGID | MHP_MEMMAP_ON_MEMORY |
>>>      +                                 MHP_SPLIT_MEMBLOCKS;
>>>                      struct resource *res;
>>>                      struct range range;
>>>       
>>>      @@ -141,7 +143,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>>                       * this as RAM automatically.
>>>                       */
>>>                      rc = add_memory_driver_managed(data->mgid, range.start,
>>>      -                               range_len(&range), kmem_name, MHP_NID_IS_MGID);
>>>      +                               range_len(&range), kmem_name, mhp_flags);
>>>       
>>>                      if (rc) {
>>>                              dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",
>>>      
>>>
>>
>> Why do we need the MHP_SPLIT_MEMBLOCKS?
> 
> I thought we still wanted either an opt-in or opt-out for the kmem
> driver to be able to do memmap_on_memory, in case there were
> performance implications or the lack of 1GiB PUDs. I haven't
> implemented that yet, but I was thinking along the lines of a sysfs
> knob exposed by kmem, that controls setting of this new
> MHP_SPLIT_MEMBLOCKS flag.

Why is MHP_MEMMAP_ON_MEMORY not sufficient for that?

> 
>>
>> In add_memory_driver_managed(), if memmap_on_memory = 1 AND is effective for a
>> single memory block, you can simply split up internally, no?
>>
>> Essentially in add_memory_resource() something like
>>
>> if (mhp_flags & MHP_MEMMAP_ON_MEMORY &&
>>       mhp_supports_memmap_on_memory(memory_block_size_bytes())) {
>>          for (cur_start = start, cur_start < start + size;
>>               cur_start += memory_block_size_bytes()) {
>>                  mhp_altmap.free = PHYS_PFN(memory_block_size_bytes());
>>                  mhp_altmap.base_pfn = PHYS_PFN(start);
>>                  params.altmap = &mhp_altmap;
>>
>>                  ret = arch_add_memory(nid, start,
>>                                        memory_block_size_bytes(), &params);
>>                  if (ret < 0) ...
>>
>>                  ret = create_memory_block_devices(start, memory_block_size_bytes(),
>>                                                    mhp_altmap.alloc, group);
>>                  if (ret) ...
>>                  
>>          }
>> } else {
>>          /* old boring stuff */
>> }
>>
>> Of course, doing it a bit cleaner, factoring out adding of mem+creating devices into
>> a helper so we can use it on the other path, avoiding repeating memory_block_size_bytes()
>> ...
> 
> My current approach was looping a level higher, on the call to
> add_memory_resource, but this looks reasonable too, and I can switch to
> this. In fact it is better as in my case I had to loop twice, once for
> the regular add_memory() path and once for the _driver_managed() path.
> Yours should avoid that.

As we only care about the altmap here, handling it for arch_add_memory() 
+ create_memory_block_devices() should be sufficient.

-- 
Cheers,

David / dhildenb