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

From: Mike Rapoport <[email-masked]>
Date: Mon, 24 May 2021 09:52:15 +0300

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 <[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
> 
> 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.

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