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Date:   Sat, 5 Mar 2022 15:13:54 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        Andrew Morton <akpm@linux-foundation.org>,
        Xuefeng Li <lixuefeng@loongson.cn>, linux-mips@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/4] MIPS: Refactor early_parse_mem() to fix mem=
 parameter
Message-ID: <YiNiEqC294xQ6PXD@kernel.org>
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 <20220304151052.GA27642@alpha.franken.de>
 <20220304153517.GA28487@alpha.franken.de>
 <alpine.DEB.2.21.2203041634040.47558@angie.orcam.me.uk>
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On Fri, Mar 04, 2022 at 05:11:44PM +0000, Maciej W. Rozycki wrote:
> On Fri, 4 Mar 2022, Thomas Bogendoerfer wrote:
> 
> > > > With this patch, when add "mem=3G" to the command-line, the
> > > > kernel boots successfully, we can see the following messages:
> > > 
> > > unfortunately this patch would break platforms without memory detection,
> > > which simply use mem=32M for memory configuration. Not sure how many
> > > rely on this mechanism. If we can make sure nobody uses it, I'm fine
> > > with your patch.
> > 
> > maybe we could add a CONFIG option, which will be selected by
> > platforms, which don't need/want this usermem thing.
> 
>  FWIW I don't understand what the issue is here beyond that we have a bug 
> that causes a system to hang when "mem=3G" is passed on the kernel command 
> line.  That is assuming that system does have contiguous RAM available for 
> the kernel to use from address 0 up to 3GiB; otherwise it's a user error 
> to tell the kernel it has that memory available (I did get bitten by that 
> myself too): garbage in, garbage out.

This is exactly the case here because the system does not have contiguous
RAM:

  [    0.000000] Early memory node ranges
  [    0.000000]   node   0: [mem 0x0000000004000000-0x000000000effffff]
  [    0.000000]   node   0: [mem 0x0000000090200000-0x00000000ffffffff]
  [    0.000000]   node   0: [mem 0x0000000120000000-0x00000001653fffff]

(from patch 3/4 in this series)

I don't see what "bug" this patch is trying to fix. Is indeed possible to
make MIPS' mem= behave like it does not arm64 and ppc, but that would break
systems that use current semantics and I recall seeing some of OpenWRT
machines using mem= to override memory map supplied by firmware. 
 
>   Maciej

-- 
Sincerely yours,
Mike.