linux-kernel - Re: [PATCH v4 2/2] dax/kmem: allow kmem to add memory with memmap_on

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Message-ID: <6e636e70-cbe5-45a7-8918-abdf196d5437@linux.ibm.com>
Date:   Wed, 4 Oct 2023 10:42:20 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "david@...hat.com" <david@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Cc:     "Hocko, Michal" <mhocko@...e.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmoyer@...hat.com" <jmoyer@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>
Subject: Re: [PATCH v4 2/2] dax/kmem: allow kmem to add memory with
 memmap_on_memory

On 10/4/23 5:18 AM, Verma, Vishal L wrote:
> On Tue, 2023-10-03 at 09:34 +0530, Aneesh Kumar K V wrote:
>> On 9/29/23 2:00 AM, Vishal Verma wrote:
>>> Large amounts of memory managed by the kmem driver may come in via CXL,
>>> and it is often desirable to have the memmap for this memory on the new
>>> memory itself.
>>>
>>> Enroll kmem-managed memory for memmap_on_memory semantics if the dax
>>> region originates via CXL. For non-CXL dax regions, retain the existing
>>> default behavior of hot adding without memmap_on_memory semantics.
>>>
>>
>> Are we not looking at doing altmap space for CXL DAX regions? Last discussion around
>> this was suggesting we look at doing this via altmap reservation so that
>> we get contigous space for device memory enabling us to map them
>> via 1G direct mapping entries? 
>>
> Hey Aneesh - was this on a previous posting or something - do you have
> a link so I can refresh myself on what the discussion was?
> 
> If it is enabling something in CXL similar to the --map=mem mode for
> pmem + device dax, that could be incremental to this.

Yes. Agree that we can do that incremental to this. 

https://lore.kernel.org/linux-mm/b6753402-2de9-25b2-36e9-eacd49752b19@redhat.com

-aneesh