Subject: Re: [PATCH v3 5/9] mm: remove CONFIG_DISCONTIGMEM

From: Mike Rapoport <[email-masked]>
Date: Sat, 12 Jun 2021 08:43:33 +0300

On Fri, Jun 11, 2021 at 01:53:48PM -0700, Stephen Brennan wrote:
> Mike Rapoport <[email-masked]> writes:
> > From: Mike Rapoport <[email-masked]>
> >
> > There are no architectures that support DISCONTIGMEM left.
> >
> > Remove the configuration option and the dead code it was guarding in the
> > generic memory management code.
> >
> > Signed-off-by: Mike Rapoport <[email-masked]>
> > ---
> >  include/asm-generic/memory_model.h | 37 ++++--------------------------
> >  include/linux/mmzone.h             |  8 ++++---
> >  mm/Kconfig                         | 25 +++-----------------
> >  mm/page_alloc.c                    | 13 -----------
> >  4 files changed, 12 insertions(+), 71 deletions(-)
> >
> > diff --git a/include/asm-generic/memory_model.h b/include/asm-generic/memory_model.h
> > index 7637fb46ba4f..a2c8ed60233a 100644
> > --- a/include/asm-generic/memory_model.h
> > +++ b/include/asm-generic/memory_model.h
> > @@ -6,47 +6,18 @@
> >  
> >  #ifndef __ASSEMBLY__
> >  
> > +/*
> > + * supports 3 memory models.
> > + */
> 
> This comment could either be updated to reflect 2 memory models, or
> removed entirely.

I counted SPARSE and SPARSE_VMEMMAP as 2.

The code below has three clauses: one for FLATMEM, one for SPARSE and one
for VMEMMAP.
 
> Thanks,
> Stephen
> 
> >  #if defined(CONFIG_FLATMEM)
> >  
> >  #ifndef ARCH_PFN_OFFSET
> >  #define ARCH_PFN_OFFSET		(0UL)
> >  #endif
> >  
> > -#elif defined(CONFIG_DISCONTIGMEM)
> > -
> > -#ifndef arch_pfn_to_nid
> > -#define arch_pfn_to_nid(pfn)	pfn_to_nid(pfn)
> > -#endif
> > -
> > -#ifndef arch_local_page_offset
> > -#define arch_local_page_offset(pfn, nid)	\
> > -	((pfn) - NODE_DATA(nid)->node_start_pfn)
> > -#endif
> > -
> > -#endif /* CONFIG_DISCONTIGMEM */
> > -
> > -/*
> > - * supports 3 memory models.
> > - */
> > -#if defined(CONFIG_FLATMEM)
> > -
> >  #define __pfn_to_page(pfn)	(mem_map + ((pfn) - ARCH_PFN_OFFSET))
> >  #define __page_to_pfn(page)	((unsigned long)((page) - mem_map) + \
> >  				 ARCH_PFN_OFFSET)
> > -#elif defined(CONFIG_DISCONTIGMEM)
> > -
> > -#define __pfn_to_page(pfn)			\
> > -({	unsigned long __pfn = (pfn);		\
> > -	unsigned long __nid = arch_pfn_to_nid(__pfn);  \
> > -	NODE_DATA(__nid)->node_mem_map + arch_local_page_offset(__pfn, __nid);\
> > -})
> > -
> > -#define __page_to_pfn(pg)						\
> > -({	const struct page *__pg = (pg);					\
> > -	struct pglist_data *__pgdat = NODE_DATA(page_to_nid(__pg));	\
> > -	(unsigned long)(__pg - __pgdat->node_mem_map) +			\
> > -	 __pgdat->node_start_pfn;					\
> > -})
> >  
> >  #elif defined(CONFIG_SPARSEMEM_VMEMMAP)
> >  
> > @@ -70,7 +41,7 @@
> >  	struct mem_section *__sec = __pfn_to_section(__pfn);	\
> >  	__section_mem_map_addr(__sec) + __pfn;		\
> >  })
> > -#endif /* CONFIG_FLATMEM/DISCONTIGMEM/SPARSEMEM */
> > +#endif /* CONFIG_FLATMEM/SPARSEMEM */
> >  
> >  /*
> >   * Convert a physical address to a Page Frame Number and back
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 0d53eba1c383..700032e99419 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -738,10 +738,12 @@ struct zonelist {
> >  	struct zoneref _zonerefs[MAX_ZONES_PER_ZONELIST + 1];
> >  };
> >  
> > -#ifndef CONFIG_DISCONTIGMEM
> > -/* The array of struct pages - for discontigmem use pgdat->lmem_map */
> > +/*
> > + * The array of struct pages for flatmem.
> > + * It must be declared for SPARSEMEM as well because there are configurations
> > + * that rely on that.
> > + */
> >  extern struct page *mem_map;
> > -#endif
> >  
> >  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >  struct deferred_split {
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 02d44e3420f5..218b96ccc84a 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -19,7 +19,7 @@ choice
> >  
> >  config FLATMEM_MANUAL
> >  	bool "Flat Memory"
> > -	depends on !(ARCH_DISCONTIGMEM_ENABLE || ARCH_SPARSEMEM_ENABLE) || ARCH_FLATMEM_ENABLE
> > +	depends on !ARCH_SPARSEMEM_ENABLE || ARCH_FLATMEM_ENABLE
> >  	help
> >  	  This option is best suited for non-NUMA systems with
> >  	  flat address space. The FLATMEM is the most efficient
> > @@ -32,21 +32,6 @@ config FLATMEM_MANUAL
> >  
> >  	  If unsure, choose this option (Flat Memory) over any other.
> >  
> > -config DISCONTIGMEM_MANUAL
> > -	bool "Discontiguous Memory"
> > -	depends on ARCH_DISCONTIGMEM_ENABLE
> > -	help
> > -	  This option provides enhanced support for discontiguous
> > -	  memory systems, over FLATMEM.  These systems have holes
> > -	  in their physical address spaces, and this option provides
> > -	  more efficient handling of these holes.
> > -
> > -	  Although "Discontiguous Memory" is still used by several
> > -	  architectures, it is considered deprecated in favor of
> > -	  "Sparse Memory".
> > -
> > -	  If unsure, choose "Sparse Memory" over this option.
> > -
> >  config SPARSEMEM_MANUAL
> >  	bool "Sparse Memory"
> >  	depends on ARCH_SPARSEMEM_ENABLE
> > @@ -62,17 +47,13 @@ config SPARSEMEM_MANUAL
> >  
> >  endchoice
> >  
> > -config DISCONTIGMEM
> > -	def_bool y
> > -	depends on (!SELECT_MEMORY_MODEL && ARCH_DISCONTIGMEM_ENABLE) || DISCONTIGMEM_MANUAL
> > -
> >  config SPARSEMEM
> >  	def_bool y
> >  	depends on (!SELECT_MEMORY_MODEL && ARCH_SPARSEMEM_ENABLE) || SPARSEMEM_MANUAL
> >  
> >  config FLATMEM
> >  	def_bool y
> > -	depends on (!DISCONTIGMEM && !SPARSEMEM) || FLATMEM_MANUAL
> > +	depends on !SPARSEMEM || FLATMEM_MANUAL
> >  
> >  config FLAT_NODE_MEM_MAP
> >  	def_bool y
> > @@ -85,7 +66,7 @@ config FLAT_NODE_MEM_MAP
> >  #
> >  config NEED_MULTIPLE_NODES
> >  	def_bool y
> > -	depends on DISCONTIGMEM || NUMA
> > +	depends on NUMA
> >  
> >  #
> >  # SPARSEMEM_EXTREME (which is the default) does some bootmem
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index aaa1655cf682..6fc22482eaa8 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -331,20 +331,7 @@ compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS] = {
> >  
> >  int min_free_kbytes = 1024;
> >  int user_min_free_kbytes = -1;
> > -#ifdef CONFIG_DISCONTIGMEM
> > -/*
> > - * DiscontigMem defines memory ranges as separate pg_data_t even if the ranges
> > - * are not on separate NUMA nodes. Functionally this works but with
> > - * watermark_boost_factor, it can reclaim prematurely as the ranges can be
> > - * quite small. By default, do not boost watermarks on discontigmem as in
> > - * many cases very high-order allocations like THP are likely to be
> > - * unsupported and the premature reclaim offsets the advantage of long-term
> > - * fragmentation avoidance.
> > - */
> > -int watermark_boost_factor __read_mostly;
> > -#else
> >  int watermark_boost_factor __read_mostly = 15000;
> > -#endif
> >  int watermark_scale_factor = 10;
> >  
> >  static unsigned long nr_kernel_pages __initdata;
> > -- 
> > 2.28.0
> 
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-- 
Sincerely yours,
Mike.

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