Re: [PATCH] mm: Check if section present during memory block (un)registering

[Andrew Morton]

On Tue, 25 Aug 2015 15:41:16 -0700 Yinghai Lu <yinghai@xxxxxxxxxx> wrote:

> Tony found on his setup, if memory block size 512M will cause crash
> during booting.
> 
>  BUG: unable to handle kernel paging request at ffffea0074000020
>  IP: [<ffffffff81670527>] get_nid_for_pfn+0x17/0x40
>  PGD 128ffcb067 PUD 128ffc9067 PMD 0 
>  Oops: 0000 [#1] SMP 
>  Modules linked in:
>  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.2.0-rc8 #1
> ...
>  Call Trace:
>   [<ffffffff81453b56>] ? register_mem_sect_under_node+0x66/0xe0
>   [<ffffffff81453eeb>] register_one_node+0x17b/0x240
>   [<ffffffff81b1f1ed>] ? pci_iommu_alloc+0x6e/0x6e
>   [<ffffffff81b1f229>] topology_init+0x3c/0x95
>   [<ffffffff8100213d>] do_one_initcall+0xcd/0x1f0
> 
> The system has non continuous RAM address:
>  BIOS-e820: [mem 0x0000001300000000-0x0000001cffffffff] usable
>  BIOS-e820: [mem 0x0000001d70000000-0x0000001ec7ffefff] usable
>  BIOS-e820: [mem 0x0000001f00000000-0x0000002bffffffff] usable
>  BIOS-e820: [mem 0x0000002c18000000-0x0000002d6fffefff] usable
>  BIOS-e820: [mem 0x0000002e00000000-0x00000039ffffffff] usable
> 
> So there are start sections in memory block not present.
> For example:
> memory block : [0x2c18000000, 0x2c20000000) 512M
> first three sections are not present.
> 
> Current register_mem_sect_under_node() assume first section is present,
> but memory block section number range [start_section_nr, end_section_nr]
> would include not present section.
> 
> For arch that support vmemmap, we don't setup memmap for struct page area
> within not present sections area.
> 
> So skip the pfn range that belong to not present section.
> 
> Als fixes unregister_mem_sect_under_nodes().

It appears this should be backported into -stable kernels, yes?  Do you
know which kernel versions need the fix?

> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -390,8 +390,14 @@ int register_mem_sect_under_node(struct memory_block *mem_blk, int nid)
>  	sect_end_pfn = section_nr_to_pfn(mem_blk->end_section_nr);
>  	sect_end_pfn += PAGES_PER_SECTION - 1;
>  	for (pfn = sect_start_pfn; pfn <= sect_end_pfn; pfn++) {
> -		int page_nid;
> +		int page_nid, scn_nr;
>  
> +		scn_nr = pfn_to_section_nr(pfn);
> +		if (!present_section_nr(scn_nr)) {
> +			pfn = round_down(pfn + PAGES_PER_SECTION,
> +					 PAGES_PER_SECTION) - 1;
> +			continue;
> +		}

Can we please add a comment here telling readers why this is being
done?  What scenario is being detected and how it comes about.

>  		page_nid = get_nid_for_pfn(pfn);
>  		if (page_nid < 0)
>  			continue;

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