Re: [PATCH v2 10/13] arc: use FLATMEM with freeing of unused memory map instead of DISCONTIGMEM

[Vineet Gupta]

Hi Mike,

On 11/1/20 9:04 AM, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
>
> Currently ARC uses DISCONTIGMEM to cope with sparse physical memory address
> space on systems with 2 memory banks. While DISCONTIGMEM avoids wasting
> memory on unpopulated memory map, it adds both memory and CPU overhead
> relatively to FLATMEM. Moreover, DISCONTINGMEM is generally considered
> deprecated.
>
> The obvious replacement for DISCONTIGMEM would be SPARSEMEM, but it is also
> less efficient than FLATMEM in pfn_to_page() and page_to_pfn() conversions.
> Besides it requires tuning of SECTION_SIZE which is not trivial for
> possible ARC memory configuration.
>
> Since the memory map for both banks is always allocated from the "lowmem"
> bank, it is possible to use FLATMEM for two-bank configuration and simply
> free the unused hole in the memory map. All is required for that is to
> provide ARC-specific pfn_valid() that will take into account actual
> physical memory configuration and define HAVE_ARCH_PFN_VALID.
>
> The resulting kernel image configured with defconfig + HIGHMEM=y is
> smaller:
>
> $ size a/vmlinux b/vmlinux
>     text    data     bss     dec     hex filename
> 4673503 1245456  279756 6198715  5e95bb a/vmlinux
> 4658706 1246864  279756 6185326  5e616e b/vmlinux
>
> $ ./scripts/bloat-o-meter a/vmlinux b/vmlinux
> add/remove: 28/30 grow/shrink: 42/399 up/down: 10986/-29025 (-18039)
> ...
> Total: Before=4709315, After=4691276, chg -0.38%
>
> Booting nSIM with haps_ns.dts results in the following memory usage
> reports:
>
> a:
> Memory: 1559104K/1572864K available (3531K kernel code, 595K rwdata, 752K rodata, 136K init, 275K bss, 13760K reserved, 0K cma-reserved, 1048576K highmem)
>
> b:
> Memory: 1559112K/1572864K available (3519K kernel code, 594K rwdata, 752K rodata, 136K init, 280K bss, 13752K reserved, 0K cma-reserved, 1048576K highmem)
>
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

Sorry this fell through the cracks. Do you have a branch I can checkout 
and do a quick test.

Thx,
-Vineet

> ---
>   arch/arc/Kconfig            |  3 ++-
>   arch/arc/include/asm/page.h | 20 +++++++++++++++++---
>   arch/arc/mm/init.c          | 29 ++++++++++++++++++++++-------
>   3 files changed, 41 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index 0a89cc9def65..c874f8ab0341 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -67,6 +67,7 @@ config GENERIC_CSUM
>   
>   config ARCH_DISCONTIGMEM_ENABLE
>   	def_bool n
> +	depends on BROKEN
>   
>   config ARCH_FLATMEM_ENABLE
>   	def_bool y
> @@ -506,7 +507,7 @@ config LINUX_RAM_BASE
>   
>   config HIGHMEM
>   	bool "High Memory Support"
> -	select ARCH_DISCONTIGMEM_ENABLE
> +	select HAVE_ARCH_PFN_VALID
>   	help
>   	  With ARC 2G:2G address split, only upper 2G is directly addressable by
>   	  kernel. Enable this to potentially allow access to rest of 2G and PAE
> diff --git a/arch/arc/include/asm/page.h b/arch/arc/include/asm/page.h
> index b0dfed0f12be..23e41e890eda 100644
> --- a/arch/arc/include/asm/page.h
> +++ b/arch/arc/include/asm/page.h
> @@ -82,11 +82,25 @@ typedef pte_t * pgtable_t;
>    */
>   #define virt_to_pfn(kaddr)	(__pa(kaddr) >> PAGE_SHIFT)
>   
> -#define ARCH_PFN_OFFSET		virt_to_pfn(CONFIG_LINUX_RAM_BASE)
> +/*
> + * When HIGHMEM is enabled we have holes in the memory map so we need
> + * pfn_valid() that takes into account the actual extents of the physical
> + * memory
> + */
> +#ifdef CONFIG_HIGHMEM
> +
> +extern unsigned long arch_pfn_offset;
> +#define ARCH_PFN_OFFSET		arch_pfn_offset
> +
> +extern int pfn_valid(unsigned long pfn);
> +#define pfn_valid		pfn_valid
>   
> -#ifdef CONFIG_FLATMEM
> +#else /* CONFIG_HIGHMEM */
> +
> +#define ARCH_PFN_OFFSET		virt_to_pfn(CONFIG_LINUX_RAM_BASE)
>   #define pfn_valid(pfn)		(((pfn) - ARCH_PFN_OFFSET) < max_mapnr)
> -#endif
> +
> +#endif /* CONFIG_HIGHMEM */
>   
>   /*
>    * __pa, __va, virt_to_page (ALERT: deprecated, don't use them)
> diff --git a/arch/arc/mm/init.c b/arch/arc/mm/init.c
> index 3a35b82a718e..ce07e697916c 100644
> --- a/arch/arc/mm/init.c
> +++ b/arch/arc/mm/init.c
> @@ -28,6 +28,8 @@ static unsigned long low_mem_sz;
>   static unsigned long min_high_pfn, max_high_pfn;
>   static phys_addr_t high_mem_start;
>   static phys_addr_t high_mem_sz;
> +unsigned long arch_pfn_offset;
> +EXPORT_SYMBOL(arch_pfn_offset);
>   #endif
>   
>   #ifdef CONFIG_DISCONTIGMEM
> @@ -98,16 +100,11 @@ void __init setup_arch_memory(void)
>   	init_mm.brk = (unsigned long)_end;
>   
>   	/* first page of system - kernel .vector starts here */
> -	min_low_pfn = ARCH_PFN_OFFSET;
> +	min_low_pfn = virt_to_pfn(CONFIG_LINUX_RAM_BASE);
>   
>   	/* Last usable page of low mem */
>   	max_low_pfn = max_pfn = PFN_DOWN(low_mem_start + low_mem_sz);
>   
> -#ifdef CONFIG_FLATMEM
> -	/* pfn_valid() uses this */
> -	max_mapnr = max_low_pfn - min_low_pfn;
> -#endif
> -
>   	/*------------- bootmem allocator setup -----------------------*/
>   
>   	/*
> @@ -153,7 +150,9 @@ void __init setup_arch_memory(void)
>   	 * DISCONTIGMEM in turns requires multiple nodes. node 0 above is
>   	 * populated with normal memory zone while node 1 only has highmem
>   	 */
> +#ifdef CONFIG_DISCONTIGMEM
>   	node_set_online(1);
> +#endif
>   
>   	min_high_pfn = PFN_DOWN(high_mem_start);
>   	max_high_pfn = PFN_DOWN(high_mem_start + high_mem_sz);
> @@ -161,8 +160,15 @@ void __init setup_arch_memory(void)
>   	max_zone_pfn[ZONE_HIGHMEM] = min_low_pfn;
>   
>   	high_memory = (void *)(min_high_pfn << PAGE_SHIFT);
> +
> +	arch_pfn_offset = min(min_low_pfn, min_high_pfn);
>   	kmap_init();
> -#endif
> +
> +#else /* CONFIG_HIGHMEM */
> +	/* pfn_valid() uses this when FLATMEM=y and HIGHMEM=n */
> +	max_mapnr = max_low_pfn - min_low_pfn;
> +
> +#endif /* CONFIG_HIGHMEM */
>   
>   	free_area_init(max_zone_pfn);
>   }
> @@ -190,3 +196,12 @@ void __init mem_init(void)
>   	highmem_init();
>   	mem_init_print_info(NULL);
>   }
> +
> +#ifdef CONFIG_HIGHMEM
> +int pfn_valid(unsigned long pfn)
> +{
> +	return (pfn >= min_high_pfn && pfn <= max_high_pfn) ||
> +		(pfn >= min_low_pfn && pfn <= max_low_pfn);
> +}
> +EXPORT_SYMBOL(pfn_valid);
> +#endif