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Date:   Tue, 5 Jan 2021 11:13:30 +0200
From:   Mike Rapoport <rppt@linux.ibm.com>
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Guillaume Tucker <guillaume.tucker@collabora.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        kernelci-results-staging@groups.io,
        "kernelci-results@groups.io" <kernelci-results@groups.io>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Mike Rapoport <rppt@kernel.org>, Baoquan He <bhe@redhat.com>
Subject: Re: kernelci/staging-next bisection: sleep.login on
 rk3288-rock2-square #2286-staging
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On Sun, Jan 03, 2021 at 03:09:14PM -0500, Andrea Arcangeli wrote:
> Hello Mike,
> 
> On Sun, Jan 03, 2021 at 03:47:53PM +0200, Mike Rapoport wrote:
> > Thanks for the logs, it seems that implicitly adding reserved regions to
> > memblock.memory wasn't that bright idea :)
> 
> Would it be possible to somehow clean up the hack then?
> 
> The only difference between the clean solution and the hack is that
> the hack intended to achieved the exact same, but without adding the
> reserved regions to memblock.memory.

I didn't consider adding reserved regions to memblock.memory as a clean
solution, this was still a hack, but I didn't think that things are that
fragile.

I still think we cannot rely on memblock.reserved to detect
memory/zone/node sizes and the boot failure reported here confirms this.
 
> The comment on that problematic area says the reserved area cannot be
> used for DMA because of some unexplained hw issue, and that doing so
> prevents booting, but since the area got reserved, even with the clean
> solution, it shouldn't have never been used for DMA?
>
> So I can only imagine that the physical memory region is way more
> problematic than just for DMA. It sounds like that anything that
> touches it, including the CPU, will hang the system, not just DMA. It
> sounds somewhat similar to the other e820 direct mapping issue on x86?

My understanding is that the boot failed because when I implicitly added
the reserved region to memblock.memory the memory size seen by
free_area_init() jumped from 2G to 4G because the reserved area was close
to 4G. The very first allocation would get a chunk from slightly below of
4G and as there is no real memory there, the kernel would crash.
 
> If you want to test the hack on the arm board to check if it boots you
> can use the below commit:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=c3ea2633015104ce0df33dcddbc36f57de1392bc

My take is your solution would boot with this memory configuration, but I
still don't think that using memblock.reserved for zone/node sizing is
correct.

> Thanks,
> Andrea
> 

-- 
Sincerely yours,
Mike.