Re: [RFC V2] mm: Enable generic pfn_valid() to handle early sections with memmap holes

From: Mike Rapoport
Date: Mon May 24 2021 - 02:52:28 EST

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@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 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@xxxxxxxxxx/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@xxxxxxx/
> >
> > 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.

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.