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Date:   Thu, 13 May 2021 09:16:48 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Mike Rapoport <rppt@kernel.org>,
        Miles Chen <miles.chen@mediatek.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org,
        wsd_upstream@mediatek.com, k-hagio-ab@nec.com
Subject: Re: [PATCH] mm/sparse: fix check_usemap_section_nr warnings
Message-ID: <20210513011648.GA6776@MiWiFi-R3L-srv>
References: <20210511093114.15123-1-miles.chen@mediatek.com>
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 <1620797600.14730.7.camel@mtkswgap22>
 <YJuh90iYeZ8F4Ain@kernel.org>
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On 05/12/21 at 12:37pm, Mike Rapoport wrote:
> On Wed, May 12, 2021 at 01:33:20PM +0800, Miles Chen wrote:
> > On Tue, 2021-05-11 at 13:24 +0300, Mike Rapoport wrote:
> > > On Tue, May 11, 2021 at 05:31:14PM +0800, Miles Chen wrote:
> > > > In current implementation of node_data, if CONFIG_NEED_MULTIPLE_NODES=y,
> > > > node_data is allocated by kzmalloc. If CONFIG_NEED_MULTIPLE_NODES=n,
> > > > we use a global variable named "contig_page_data".
> > > > 
> > > > If CONFIG_DEBUG_VIRTUAL is not enabled. __pa() can handle both kzalloc and
> > > > symbol cases. But if CONFIG_DEBUG_VIRTUAL is set, we will have the
> > > > "virt_to_phys used for non-linear address" warning when booting.
> > > 
> > > Maybe we'll just allocate pgdat for CONFIG_NEED_MULTIPLE_NODES=n (which is
> > > essentially !NUMA) case in, say, free_area_init()?
> > 
> > 
> > thanks for your comment.
> > 
> > I check the source tree and found that contig_page_data is used by
> > crash_core.c as a symbol. I am not familiar with crash_core but I guess
> > allocate pgdat may break this crash_core users.
> > 
> > For example: some userspace scripts want to read the address of
> > contig_page_data symbol from a corefile.
> > 
> > kernel/crash_core.c:460:        VMCOREINFO_SYMBOL(contig_page_data);
> > 
> > #ifndef CONFIG_NEED_MULTIPLE_NODES
> >         VMCOREINFO_SYMBOL(mem_map);
> >         VMCOREINFO_SYMBOL(contig_page_data);
> > #endif
> 
> My understanding is that VMCOREINFO_SYMBOL() should correspond to actual
> symbol. If there is no contig_page_data symbol, there is no need for
> VMCOREINFO_SYMBOL() either.

Yeah, it's exported for makedumpfile and crash utility to parse and get
the memory layout of the corrupted kernel. If removing it, makedumpfile
will get it from node_data[]. Looks like a good idea to unify code for
numa|!numa on pglist_data instances.

Add Kazu to CC since he maintain makedumpfile and Crash utilities.

My concern is that that only happens on arm/arm64/riscv, does it mean the
warning is not necessary, so can be removed? Or we need to check if
CONFIG_DEBUG_VIRTUAL doesn't work well in this case.

Thanks
Baoquan