Re: [PATCH] mm/sparse: fix check_usemap_section_nr warnings

[Miles Chen <miles.chen@mediatek.com>]

On Thu, 2021-05-13 at 09:16 +0800, Baoquan He wrote:
> 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.

thanks for adding the experts in. (I searched the source code of crash
last night and found that contig_page_data is used in memory.c)

I will move the allocation and initialization to free_area_init() and
submit patch v2.


Miles

> 
> 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
>