linux-kernel - Re: collision between ZONE_MOVABLE and memblock allocations

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Message-ID: <61234a18-7797-722e-ae14-1aa352c4d850@redhat.com>
Date:   Wed, 26 Jul 2023 10:31:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ross Zwisler <zwisler@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: collision between ZONE_MOVABLE and memblock allocations

>> 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