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

Mike Rapoport
2020-11-17T06:57:08Z

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 <rppt@linux.ibm.com>
> >
> > 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: BeforeG09315, AfterF91276, 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@linux.ibm.com>
>
> 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.