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

From: Mike Rapoport <[email-masked]>
Date: Tue, 17 Nov 2020 08:57:08 +0200

On Tue, Nov 17, 2020 at 06:40:16AM +0000, Vineet Gupta wrote:
> Hi Mike,
> 
> On 11/1/20 9:04 AM, Mike Rapoport wrote:
> > From: Mike Rapoport <[email-masked]>
> >
> > 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 <[email-masked]>
> 
> Sorry this fell through the cracks. Do you have a branch I can checkout 
> and do a quick test.

It's in mmotm and in my tree:
https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git memory-models/rm-discontig/v0

> 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
> 

-- 
Sincerely yours,
Mike.

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