Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <61234a18-7797-722e-ae14-1aa352c4d850@redhat.com>
Date:   Wed, 26 Jul 2023 10:31:02 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
Content-Language: en-US
To:     Ross Zwisler <zwisler@google.com>
Cc:     Michal Hocko <mhocko@suse.com>, Mike Rapoport <rppt@kernel.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>
References: <20230718220106.GA3117638@google.com>
 <ZLd/WEZTH5rlwYjP@dhcp22.suse.cz> <20230719075952.GH1901145@kernel.org>
 <ZLeZaa5LMH1c2zQ3@dhcp22.suse.cz>
 <9770454d-f840-c7cf-314e-ce81839393e3@redhat.com>
 <20230719230515.GA3654720@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: collision between ZONE_MOVABLE and memblock allocations
In-Reply-To: <20230719230515.GA3654720@google.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

>> As an alternative, we might use the "memmap=nn[KMG]!ss[KMG]" [1] parameter
>> to mark some memory as protected.
>>
>> That memory can then be configured as devdax device and online to
>> ZONE_MOVABLE (dev/dax).
>>
>> [1] https://docs.pmem.io/persistent-memory/getting-started-guide/creating-development-environments/linux-environments/linux-memmap
> 
> I've previously been reconfiguring devdax memory like this:
> 
>    ndctl create-namespace --reconfig=namespace0.0 -m devdax -f
>    daxctl reconfigure-device --mode=system-ram dax0.0
> 
> Is this how you've been doing it, or is there something else I should
> consider?

No, exactly like that.

> 
> I just sent mail to Michal outlining my use case, hopefully it makes sense.

Yes, thanks for sharing, I'll dig deeper into that next.

> 
> I had thought about using 'memmap=' in the first kernel and the worry was that
> I'd have to support many different machines with different memory
> configurations, and have to hard-code memory offsets and lengths for the
> various memmap= kernel command line parameters. 

Indeed.

> If I can make ZONE_MOVABLE
> work that's preferable because the kernel will choose the correct usermem-only
> region for me, and then I can just use that region for the crash kernel and
> 3rd kernel boots.

It really sounds like you might be better off using CMA or 
alloc_contig_pages().

The latter is unreliable, though, and the memory cannot be used for 
other purposes once alloc_contig_pages() succeeded

See arch/powerpc/platforms/powernv/memtrace.c one one user that needs to 
set aside a lot of memory to store HW traces.

-- 
Cheers,

David / dhildenb