Re: [RFC] mm: Enable generic pfn_valid() to handle early sections with memmap holes

From: Mike Rapoport
Date: Mon Mar 08 2021 - 03:56:33 EST

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Hi Anshuman,

On Mon, Mar 08, 2021 at 08:57:53AM +0530, Anshuman Khandual wrote:
> Platforms like arm and arm64 have redefined pfn_valid() because their early
> memory sections might have contained memmap holes caused by memblock areas
> tagged with MEMBLOCK_NOMAP, which should be skipped while validating a pfn
> for struct page backing. This scenario could be captured with a new option
> CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES and then generic pfn_valid() can be
> improved to accommodate such platforms. This reduces overall code footprint
> and also improves maintainability.

I wonder whether arm64 would still need to free parts of its memmap after
the section size was reduced. Maybe the pain of arm64::pfn_valid() is not
worth the memory savings anymore?

> Commit 4f5b0c178996 ("arm, arm64: move free_unused_memmap() to generic mm")
> had used CONFIG_HAVE_ARCH_PFN_VALID to gate free_unused_memmap(), which in
> turn had expanded its scope to new platforms like arc and m68k. Rather lets
> restrict back the scope for free_unused_memmap() to arm and arm64 platforms
> using this new config option i.e CONFIG_HAVE_EARLY_SECTION_MEMMAP.

The whole point of 4f5b0c178996 was to let arc and m68k to free unused
memory map with FLATMEM so they won't need DISCONTIGMEM or SPARSEMEM. So
whatever implementation there will be for arm/arm64, please keep arc and
m68k functionally intact.

> While here, it exports the symbol memblock_is_map_memory() to build drivers
> that depend on pfn_valid() but does not have the required visibility. After
> this new config is in place, just drop CONFIG_HAVE_ARCH_PFN_VALID from both
> arm and arm64 platforms.
>
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
> This applies on 5.12-rc2 along with arm64 pfn_valid() fix patches [1] and
> has been lightly tested on the arm64 platform. The idea to represent this
> unique situation on the arm and arm64 platforms with a config option was
> proposed by David H during an earlier discussion [2]. This still does not
> build on arm platform due to pfn_valid() resolution errors. Nonetheless
> wanted to get some early feedback whether the overall approach here, is
> acceptable or not.
>
> [1] https://patchwork.kernel.org/project/linux-mm/list/?series=442433
> [2] https://lore.kernel.org/linux-arm-kernel/4b282848-d2d7-6156-4726-ce974b2dff41@xxxxxxxxxx/
>
> arch/arm/Kconfig | 2 +-
> arch/arm/include/asm/page.h | 4 ----
> arch/arm/mm/init.c | 13 -----------
> arch/arm64/Kconfig | 2 +-
> arch/arm64/include/asm/page.h | 2 --
> arch/arm64/mm/init.c | 41 -----------------------------------
> include/linux/mmzone.h | 24 +++++++++++++++++++-
> mm/Kconfig | 10 +++++++++
> mm/memblock.c | 3 ++-
> 9 files changed, 37 insertions(+), 64 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 853aab5ab327..8b1d3089baa6 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -71,7 +71,6 @@ config ARM
> select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
> select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL
> select HAVE_ARCH_MMAP_RND_BITS if MMU
> - select HAVE_ARCH_PFN_VALID
> select HAVE_ARCH_SECCOMP
> select HAVE_ARCH_SECCOMP_FILTER if AEABI && !OABI_COMPAT
> select HAVE_ARCH_THREAD_STRUCT_WHITELIST
> @@ -84,6 +83,7 @@ config ARM
> select HAVE_DMA_CONTIGUOUS if MMU
> select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
> select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
> + select HAVE_EARLY_SECTION_MEMMAP_HOLES
> select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
> select HAVE_EXIT_THREAD
> select HAVE_FAST_GUP if ARM_LPAE
> diff --git a/arch/arm/include/asm/page.h b/arch/arm/include/asm/page.h
> index 11b058a72a5b..7e3189083bd7 100644
> --- a/arch/arm/include/asm/page.h
> +++ b/arch/arm/include/asm/page.h
> @@ -153,10 +153,6 @@ extern void copy_page(void *to, const void *from);
>
> typedef struct page *pgtable_t;
>
> -#ifdef CONFIG_HAVE_ARCH_PFN_VALID
> -extern int pfn_valid(unsigned long);
> -#endif
> -
> #include <asm/memory.h>
>
> #endif /* !__ASSEMBLY__ */
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 828a2561b229..9131ef4e599e 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -121,19 +121,6 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max_low,
> free_area_init(max_zone_pfn);
> }
>
> -#ifdef CONFIG_HAVE_ARCH_PFN_VALID
> -int pfn_valid(unsigned long pfn)
> -{
> - phys_addr_t addr = __pfn_to_phys(pfn);
> -
> - if (__phys_to_pfn(addr) != pfn)
> - return 0;
> -
> - return memblock_is_map_memory(addr);
> -}
> -EXPORT_SYMBOL(pfn_valid);
> -#endif
> -
> static bool arm_memblock_steal_permitted = true;
>
> phys_addr_t __init arm_memblock_steal(phys_addr_t size, phys_addr_t align)
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1f212b47a48a..2ee48bdf9dc1 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -144,7 +144,6 @@ config ARM64
> select HAVE_ARCH_KGDB
> select HAVE_ARCH_MMAP_RND_BITS
> select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> - select HAVE_ARCH_PFN_VALID
> select HAVE_ARCH_PREL32_RELOCATIONS
> select HAVE_ARCH_SECCOMP_FILTER
> select HAVE_ARCH_STACKLEAK
> @@ -167,6 +166,7 @@ config ARM64
> if $(cc-option,-fpatchable-function-entry=2)
> select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
> if DYNAMIC_FTRACE_WITH_REGS
> + select HAVE_EARLY_SECTION_MEMMAP_HOLES
> select HAVE_EFFICIENT_UNALIGNED_ACCESS
> select HAVE_FAST_GUP
> select HAVE_FTRACE_MCOUNT_RECORD
> diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> index 012cffc574e8..635a030a3826 100644
> --- a/arch/arm64/include/asm/page.h
> +++ b/arch/arm64/include/asm/page.h
> @@ -37,8 +37,6 @@ void copy_highpage(struct page *to, struct page *from);
>
> typedef struct page *pgtable_t;
>
> -extern int pfn_valid(unsigned long);
> -
> #include <asm/memory.h>
>
> #endif /* !__ASSEMBLY__ */
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 3685e12aba9b..2cba4347aef2 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -217,47 +217,6 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
> free_area_init(max_zone_pfns);
> }
>
> -int pfn_valid(unsigned long pfn)
> -{
> - phys_addr_t addr = PFN_PHYS(pfn);
> -
> - /*
> - * Ensure the upper PAGE_SHIFT bits are clear in the
> - * pfn. Else it might lead to false positives when
> - * some of the upper bits are set, but the lower bits
> - * match a valid pfn.
> - */
> - if (PHYS_PFN(addr) != pfn)
> - return 0;
> -
> -#ifdef CONFIG_SPARSEMEM
> -{
> - struct mem_section *ms;
> -
> - if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> - return 0;
> -
> - ms = __pfn_to_section(pfn);
> - if (!valid_section(ms))
> - return 0;
> -
> - /*
> - * ZONE_DEVICE memory does not have the memblock entries.
> - * memblock_is_map_memory() check for ZONE_DEVICE based
> - * addresses will always fail. Even the normal hotplugged
> - * memory will never have MEMBLOCK_NOMAP flag set in their
> - * memblock entries. Skip memblock search for all non early
> - * memory sections covering all of hotplug memory including
> - * both normal and ZONE_DEVICE based.
> - */
> - if (!early_section(ms))
> - return pfn_section_valid(ms, pfn);
> -}
> -#endif
> - return memblock_is_map_memory(addr);
> -}
> -EXPORT_SYMBOL(pfn_valid);
> -
> static phys_addr_t memory_limit = PHYS_ADDR_MAX;
>
> /*
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 47946cec7584..93532994113f 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1409,8 +1409,23 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
> }
> #endif
>
> +bool memblock_is_map_memory(phys_addr_t addr);
> +
> #ifndef CONFIG_HAVE_ARCH_PFN_VALID
> static inline int pfn_valid(unsigned long pfn)
> +{
> + phys_addr_t addr = PFN_PHYS(pfn);
> +
> + /*
> + * Ensure the upper PAGE_SHIFT bits are clear in the
> + * pfn. Else it might lead to false positives when
> + * some of the upper bits are set, but the lower bits
> + * match a valid pfn.
> + */
> + if (PHYS_PFN(addr) != pfn)
> + return 0;
> +
> +#ifdef CONFIG_SPARSEMEM
> {
> struct mem_section *ms;
>
> @@ -1423,7 +1438,14 @@ static inline int pfn_valid(unsigned long pfn)
> * Traditionally early sections always returned pfn_valid() for
> * the entire section-sized span.
> */
> - return early_section(ms) || pfn_section_valid(ms, pfn);
> + if (early_section(ms))
> + return IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) ?
> + memblock_is_map_memory(pfn << PAGE_SHIFT) : 1;
> +
> + return pfn_section_valid(ms, pfn);
> +}
> +#endif
> + return 1;
> }
> #endif
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 24c045b24b95..0ec20f661b3f 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -135,6 +135,16 @@ config HAVE_FAST_GUP
> config ARCH_KEEP_MEMBLOCK
> bool
>
> +config HAVE_EARLY_SECTION_MEMMAP_HOLES
> + depends on ARCH_KEEP_MEMBLOCK && SPARSEMEM_VMEMMAP
> + def_bool n
> + help
> + Early sections on certain platforms might have portions which are
> + not backed with struct page mapping as their memblock entries are
> + marked with MEMBLOCK_NOMAP. When subscribed, this option enables
> + specific handling for those memory sections in certain situations
> + such as pfn_valid().
> +
> # Keep arch NUMA mapping infrastructure post-init.
> config NUMA_KEEP_MEMINFO
> bool
> diff --git a/mm/memblock.c b/mm/memblock.c
> index afaefa8fc6ab..d9fa2e62ab7a 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1744,6 +1744,7 @@ bool __init_memblock memblock_is_map_memory(phys_addr_t addr)
> return false;
> return !memblock_is_nomap(&memblock.memory.regions[i]);
> }
> +EXPORT_SYMBOL(memblock_is_map_memory);
>
> int __init_memblock memblock_search_pfn_nid(unsigned long pfn,
> unsigned long *start_pfn, unsigned long *end_pfn)
> @@ -1926,7 +1927,7 @@ static void __init free_unused_memmap(void)
> unsigned long start, end, prev_end = 0;
> int i;
>
> - if (!IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) ||
> + if (!IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) ||
> IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
> return;
>
> --
> 2.20.1
>

--
Sincerely yours,
Mike.

Next message: jingle: "RE: [PATCH] Input: elan_i2c - Reduce the resume time for new dev ices"
Previous message: Yang Li: "[PATCH] perf symbols: turn dso__fprintf_symbols_by_name into void function"
In reply to: Anshuman Khandual: "Re: [RFC] mm: Enable generic pfn_valid() to handle early sections with memmap holes"
Next in thread: Anshuman Khandual: "Re: [RFC] mm: Enable generic pfn_valid() to handle early sections with memmap holes"
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