Re: [PATCH] mm/page_alloc: Skip non present sections on zone initialization

[Baoquan He]

On 12/30/19 at 12:38pm, Kirill A. Shutemov wrote:
> memmap_init_zone() can be called on the ranges with holes during the
> boot. It will skip any non-valid PFNs one-by-one. It works fine as long
> as holes are not too big.
> 
> But huge holes in the memory map causes a problem. It takes over 20
> seconds to walk 32TiB hole. x86-64 with 5-level paging allows for much
> larger holes in the memory map which would practically hang the system.
> 
> Deferred struct page init doesn't help here. It only works on the
> present ranges.
> 
> Skipping non-present sections would fix the issue.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
> 
> The situation can be emulated using the following QEMU patch:
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index ac08e6360437..f5f2258092e1 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1159,13 +1159,14 @@ void pc_memory_init(PCMachineState *pcms,
>      memory_region_add_subregion(system_memory, 0, ram_below_4g);
>      e820_add_entry(0, x86ms->below_4g_mem_size, E820_RAM);
>      if (x86ms->above_4g_mem_size > 0) {
> +        int shift = 45;
>          ram_above_4g = g_malloc(sizeof(*ram_above_4g));
>          memory_region_init_alias(ram_above_4g, NULL, "ram-above-4g", ram,
>                                   x86ms->below_4g_mem_size,
>                                   x86ms->above_4g_mem_size);
> -        memory_region_add_subregion(system_memory, 0x100000000ULL,
> +        memory_region_add_subregion(system_memory, 1ULL << shift,
>                                      ram_above_4g);
> -        e820_add_entry(0x100000000ULL, x86ms->above_4g_mem_size, E820_RAM);
> +        e820_add_entry(1ULL << shift, x86ms->above_4g_mem_size, E820_RAM);
>      }
>  
>      if (!pcmc->has_reserved_memory &&
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index cde2a16b941a..694c26947bf6 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1928,7 +1928,7 @@ uint64_t cpu_get_tsc(CPUX86State *env);
>  /* XXX: This value should match the one returned by CPUID
>   * and in exec.c */
>  # if defined(TARGET_X86_64)
> -# define TCG_PHYS_ADDR_BITS 40
> +# define TCG_PHYS_ADDR_BITS 52
>  # else
>  # define TCG_PHYS_ADDR_BITS 36
>  # endif
> 
> ---
>  mm/page_alloc.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index df62a49cd09e..442dc0244bb4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5873,6 +5873,30 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
>  	return false;
>  }
>  
> +#ifdef CONFIG_SPARSEMEM
> +/* Skip PFNs that belong to non-present sections */
> +static inline __meminit unsigned long next_pfn(unsigned long pfn)
> +{
> +	unsigned long section_nr;
> +
> +	section_nr = pfn_to_section_nr(++pfn);
> +	if (present_section_nr(section_nr))
> +		return pfn;
> +
> +	while (++section_nr <= __highest_present_section_nr) {
> +		if (present_section_nr(section_nr))
> +			return section_nr_to_pfn(section_nr);
> +	}
> +
> +	return -1;
> +}
> +#else
> +static inline __meminit unsigned long next_pfn(unsigned long pfn)
> +{
> +	return pfn++;
> +}
> +#endif
> +
>  /*
>   * Initially all pages are reserved - free ones are freed
>   * up by memblock_free_all() once the early boot process is
> @@ -5912,8 +5936,10 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  		 * function.  They do not exist on hotplugged memory.
>  		 */
>  		if (context == MEMMAP_EARLY) {
> -			if (!early_pfn_valid(pfn))
> +			if (!early_pfn_valid(pfn)) {
> +				pfn = next_pfn(pfn) - 1;

Just pass by, I think this is a necessary optimization. Wondering why
next_pfn(pfn) is not put in for loop:
-	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
+	for (pfn = start_pfn; pfn < end_pfn; pfn=next_pfn(pfn)) {


>  				continue;
> +			}
>  			if (!early_pfn_in_nid(pfn, nid))
>  				continue;

Why the other two 'continue' don't need be worried on the huge hole
case?

Thanks
Baoquan