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

From: Mike Rapoport <[email-masked]>
Date: Thu, 22 Apr 2021 12:42:58 +0300

On Thu, Apr 22, 2021 at 11:03:50AM +0200, David Hildenbrand wrote:
> On 22.04.21 09:50, 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 <[email-masked]>
> > Cc: Will Deacon <[email-masked]>
> > Cc: Andrew Morton <[email-masked]>
> > Cc: Mike Rapoport <[email-masked]>
> > Cc: David Hildenbrand <[email-masked]>
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Suggested-by: David Hildenbrand <[email-masked]>
> > Signed-off-by: Anshuman Khandual <[email-masked]>
> > ---
> > This patch applies on the latest mainline kernel after Mike's series
> > regarding arm64 based pfn_valid().
> > 
> > https://lore.kernel.org/linux-mm/[email-masked]/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/[email-masked]/
> > 
> >   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
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 24c045b24b95..db7128111874 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -135,6 +135,15 @@ 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 some memory ranges that
> > +	  are not backed with struct page mappings. When subscribed, this option
> > +	  enables special handling for those memory ranges 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 3abf2c3fea7f..93f8a9c8428d 100644
> > --- a/mm/memblock.c
> > +++ b/mm/memblock.c
> > @@ -1740,6 +1740,7 @@ bool __init_memblock memblock_is_memory(phys_addr_t addr)
> >   {
> >   	return memblock_search(&memblock.memory, addr) != -1;
> >   }
> > +EXPORT_SYMBOL(memblock_is_memory);
> >   bool __init_memblock memblock_is_map_memory(phys_addr_t addr)
> >   {
> > @@ -1931,8 +1932,11 @@ static void __init free_unused_memmap(void)
> >   	unsigned long start, end, prev_end = 0;
> >   	int i;
> > -	if (!IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) ||
> > -	    IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
> > +	if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
> > +		return;
> > +
> > +	if (!IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) &&
> > +	    !IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID))
> >   		return;
> >   	/*
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 0cdbbfbc5757..8c78b6a3d888 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -309,6 +309,11 @@ struct page *pfn_to_online_page(unsigned long pfn)
> >   	 * Save some code text when online_section() +
> >   	 * pfn_section_valid() are sufficient.
> >   	 */
> > +	if (IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES)) {
> > +		if (early_section(ms) && !memblock_is_memory(PFN_PHYS(pfn)))
> > +			return NULL;
> > +	}
> > +
> >   	if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) && !pfn_valid(pfn))
> >   		return NULL;
> > 
> 
> What about doing one step at a time and switching only over to generic
> pfn_valid() in case of CONFIG_SPARSEMEM? (meaning: freeing the memmap only
> with !CONFIG_SPARSEMEM)

The "generic" pfn_valid() is only available with CONFIG_SPARSEMEM.
With FLATMEM it's wild west:

$ git grep -w "define pfn_valid" arch/*/include/asm/ | wc -l
22

This would actually mean that we still need arm64::pfn_valid() for the
FLATMEM case.

> IOW, avoiding having to adjust generic pfn_valid()/pfn_to_online_page() at
> all. Am i missing something or should that be possible?

We are back again to the question "should arm64 free its memmap". 
If the answer is no, we don't need arm64::pfn_valid() for SPARSEMEM at all.
If the answer is yes, Anshuman's patch is way better than a custom
pfn_valid().

-- 
Sincerely yours,
Mike.

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