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Message-ID: <YKyaBi9zoTsCNrgd@kernel.org>
Date: Tue, 25 May 2021 09:32:38 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-mm@...ck.org, david@...hat.com, akpm@...ux-foundation.org,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] mm: Enable generic pfn_valid() to handle early sections
with memmap holes
On Tue, May 25, 2021 at 11:30:15AM +0530, Anshuman Khandual wrote:
>
> On 5/24/21 12:22 PM, Mike Rapoport wrote:
> > Hello Anshuman,
> >
> > On Mon, May 24, 2021 at 10:28:32AM +0530, Anshuman Khandual wrote:
> >>
> >> On 4/22/21 1:20 PM, Anshuman Khandual wrote:
> >>> Platforms like arm and arm64 have redefined pfn_valid() because their early
> >>> memory sections might have contained memmap holes after freeing parts of it
> >>> during boot, which should be skipped while validating a pfn for struct page
> >>> backing. This scenario on certain platforms where memmap is not continuous,
> >>> could be captured with a new option CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES.
> >>> Then the generic pfn_valid() can be improved to accommodate such platforms.
> >>> This reduces overall code footprint and also improves maintainability.
> >>>
> >>> free_unused_memmap() and pfn_to_online_page() have been updated to include
> >>> such cases. This also exports memblock_is_memory() for all drivers that use
> >>> pfn_valid() but lack required visibility. After the new config is in place,
> >>> drop CONFIG_HAVE_ARCH_PFN_VALID from arm64 platforms.
> >>>
> >>> Cc: Catalin Marinas <catalin.marinas@....com>
> >>> Cc: Will Deacon <will@...nel.org>
> >>> Cc: Andrew Morton <akpm@...ux-foundation.org>
> >>> Cc: Mike Rapoport <rppt@...nel.org>
> >>> Cc: David Hildenbrand <david@...hat.com>
> >>> Cc: linux-arm-kernel@...ts.infradead.org
> >>> Cc: linux-kernel@...r.kernel.org
> >>> Cc: linux-mm@...ck.org
> >>> Suggested-by: David Hildenbrand <david@...hat.com>
> >>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> >>> ---
> >>> This patch applies on the latest mainline kernel after Mike's series
> >>> regarding arm64 based pfn_valid().
> >>>
> >>> https://lore.kernel.org/linux-mm/20210422061902.21614-1-rppt@kernel.org/T/#t
> >>>
> >>> Changes in RFC V2:
> >>>
> >>> - Dropped support for arm (32 bit)
> >>> - Replaced memblock_is_map_memory() check with memblock_is_memory()
> >>> - MEMBLOCK_NOMAP memory are no longer skipped for pfn_valid()
> >>> - Updated pfn_to_online_page() per David
> >>> - Updated free_unused_memmap() to preserve existing semantics per Mike
> >>> - Exported memblock_is_memory() instead of memblock_is_map_memory()
> >>>
> >>> Changes in RFC V1:
> >>>
> >>> - https://patchwork.kernel.org/project/linux-mm/patch/1615174073-10520-1-git-send-email-anshuman.khandual@arm.com/
> >>>
> >>> arch/arm64/Kconfig | 2 +-
> >>> arch/arm64/include/asm/page.h | 1 -
> >>> arch/arm64/mm/init.c | 41 -----------------------------------
> >>> include/linux/mmzone.h | 18 ++++++++++++++-
> >>> mm/Kconfig | 9 ++++++++
> >>> mm/memblock.c | 8 +++++--
> >>> mm/memory_hotplug.c | 5 +++++
> >>> 7 files changed, 38 insertions(+), 46 deletions(-)
> >>>
> >>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >>> index b4a9b493ce72..4cdc3570ffa9 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 75ddfe671393..fcbef3eec4b2 100644
> >>> --- a/arch/arm64/include/asm/page.h
> >>> +++ b/arch/arm64/include/asm/page.h
> >>> @@ -37,7 +37,6 @@ void copy_highpage(struct page *to, struct page *from);
> >>>
> >>> typedef struct page *pgtable_t;
> >>>
> >>> -int pfn_valid(unsigned long pfn);
> >>> int pfn_is_map_memory(unsigned long pfn);
> >>>
> >>> #include <asm/memory.h>
> >>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> >>> index f431b38d0837..5731a11550d8 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_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_memory(addr);
> >>> -}
> >>> -EXPORT_SYMBOL(pfn_valid);
> >>> -
> >>> int pfn_is_map_memory(unsigned long pfn)
> >>> {
> >>> phys_addr_t addr = PFN_PHYS(pfn);
> >>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> >>> index 961f0eeefb62..18bf71665211 100644
> >>> --- a/include/linux/mmzone.h
> >>> +++ b/include/linux/mmzone.h
> >>> @@ -1421,10 +1421,22 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
> >>> *
> >>> * Return: 1 for PFNs that have memory map entries and 0 otherwise
> >>> */
> >>> +bool memblock_is_memory(phys_addr_t addr);
> >>> +
> >>> static inline int pfn_valid(unsigned long pfn)
> >>> {
> >>> + phys_addr_t addr = PFN_PHYS(pfn);
> >>> struct mem_section *ms;
> >>>
> >>> + /*
> >>> + * 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;
> >>> +
> >>> if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> >>> return 0;
> >>> ms = __nr_to_section(pfn_to_section_nr(pfn));
> >>> @@ -1434,7 +1446,11 @@ 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_memory(pfn << PAGE_SHIFT) : 1;
> >>> +
> >>> + return pfn_section_valid(ms, pfn);
> >>> }
> >>> #endif
> >>
> >> Hello David/Mike,
> >>
> >> Now that pfn_is_map_memory() usage has been decoupled from pfn_valid() and
> >> SPARSEMEM_VMEMMAP is only available memory model on arm64, wondering if we
> >> still need this HAVE_EARLY_SECTION_MEMMAP_HOLES proposal ? Please do kindly
> >> suggest. Thank you.
> >
> > Even now arm64 still frees parts of the memory map and pfn_valid() should
> > be able to tell if a part of a section is freed or not.
> >
> > For instance for the following memory configuration
> >
> > |<----section---->|<----hole---->|<----section---->|
> > +--------+--------+--------------+--------+--------+
> > | bank 0 | unused | | bank 1 | unused |
> > +--------+--------+--------------+--------+--------+
> >
> > the memory map corresponding to the "unused" areas is freed, but the generic
> > pfn_valid() will still return 1 there.
>
> But is not free_unused_memmap() return early when CONFIG_SPARSEMEM_VMEMMAP
> is enabled, which is the only option now on arm64. Then how can memmap have
> holes (from unused areas) anymore ? Am I missing something here.
Ah, you are right, I missed this detail myself :)
With CONFIG_SPARSEMEM_VMEMMAP as the only memory model for arm64, we can
simply rid of arm64::pfn_valid() without any changes to the generic
version.
> > So we either should stop freeing unused memory map on arm64, or keep
> > arm64::pfn_valid() or implement something along the lines of this patch.
> >
> > I personally don't think that the memory savings from freeing the unused
> > memory map worth the pain of maintenance and bugs happening here and there.
> >
--
Sincerely yours,
Mike.
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