The MM subsystem is trying to shrink struct page. This patchset
introduces a memory descriptor for page table tracking - struct ptdesc.

This patchset introduces ptdesc, splits ptdesc from struct page, and
converts many callers of page table constructor/destructors to use ptdescs.

Ptdesc is a foundation to further standardize page tables, and eventually
allow for dynamic allocation of page tables independent of struct page.
However, the use of pages for page table tracking is quite deeply
ingrained and varied across archictectures, so there is still a lot of
work to be done before that can happen.

This is rebased on mm-unstable.

v7:
  Drop s390 gmap ptdesc conversions - gmap is unecessary complication
    that can be dealt with later
  Be more thorough with ptdesc struct sanity checks and comments
  Rebase onto mm-unstable

Vishal Moola (Oracle) (31):
  mm: Add PAGE_TYPE_OP folio functions
  pgtable: Create struct ptdesc
  mm: add utility functions for ptdesc
  mm: Convert pmd_pgtable_page() callers to use pmd_ptdesc()
  mm: Convert ptlock_alloc() to use ptdescs
  mm: Convert ptlock_ptr() to use ptdescs
  mm: Convert pmd_ptlock_init() to use ptdescs
  mm: Convert ptlock_init() to use ptdescs
  mm: Convert pmd_ptlock_free() to use ptdescs
  mm: Convert ptlock_free() to use ptdescs
  mm: Create ptdesc equivalents for pgtable_{pte,pmd}_page_{ctor,dtor}
  powerpc: Convert various functions to use ptdescs
  x86: Convert various functions to use ptdescs
  s390: Convert various pgalloc functions to use ptdescs
  mm: Remove page table members from struct page
  pgalloc: Convert various functions to use ptdescs
  arm: Convert various functions to use ptdescs
  arm64: Convert various functions to use ptdescs
  csky: Convert __pte_free_tlb() to use ptdescs
  hexagon: Convert __pte_free_tlb() to use ptdescs
  loongarch: Convert various functions to use ptdescs
  m68k: Convert various functions to use ptdescs
  mips: Convert various functions to use ptdescs
  nios2: Convert __pte_free_tlb() to use ptdescs
  openrisc: Convert __pte_free_tlb() to use ptdescs
  riscv: Convert alloc_{pmd, pte}_late() to use ptdescs
  sh: Convert pte_free_tlb() to use ptdescs
  sparc64: Convert various functions to use ptdescs
  sparc: Convert pgtable_pte_page_{ctor, dtor}() to ptdesc equivalents
  um: Convert {pmd, pte}_free_tlb() to use ptdescs
  mm: Remove pgtable_{pmd, pte}_page_{ctor, dtor}() wrappers

 Documentation/mm/split_page_table_lock.rst    |  12 +-
 .../zh_CN/mm/split_page_table_lock.rst        |  14 +-
 arch/arm/include/asm/tlb.h                    |  12 +-
 arch/arm/mm/mmu.c                             |   7 +-
 arch/arm64/include/asm/tlb.h                  |  14 +-
 arch/arm64/mm/mmu.c                           |   7 +-
 arch/csky/include/asm/pgalloc.h               |   4 +-
 arch/hexagon/include/asm/pgalloc.h            |   8 +-
 arch/loongarch/include/asm/pgalloc.h          |  27 ++--
 arch/loongarch/mm/pgtable.c                   |   7 +-
 arch/m68k/include/asm/mcf_pgalloc.h           |  47 +++---
 arch/m68k/include/asm/sun3_pgalloc.h          |   8 +-
 arch/m68k/mm/motorola.c                       |   4 +-
 arch/mips/include/asm/pgalloc.h               |  32 ++--
 arch/mips/mm/pgtable.c                        |   8 +-
 arch/nios2/include/asm/pgalloc.h              |   8 +-
 arch/openrisc/include/asm/pgalloc.h           |   8 +-
 arch/powerpc/mm/book3s64/mmu_context.c        |  10 +-
 arch/powerpc/mm/book3s64/pgtable.c            |  32 ++--
 arch/powerpc/mm/pgtable-frag.c                |  56 +++----
 arch/riscv/include/asm/pgalloc.h              |   8 +-
 arch/riscv/mm/init.c                          |  16 +-
 arch/s390/include/asm/pgalloc.h               |   4 +-
 arch/s390/include/asm/tlb.h                   |   4 +-
 arch/s390/mm/pgalloc.c                        | 128 +++++++--------
 arch/sh/include/asm/pgalloc.h                 |   9 +-
 arch/sparc/mm/init_64.c                       |  17 +-
 arch/sparc/mm/srmmu.c                         |   5 +-
 arch/um/include/asm/pgalloc.h                 |  18 +--
 arch/x86/mm/pgtable.c                         |  47 +++---
 arch/x86/xen/mmu_pv.c                         |   2 +-
 include/asm-generic/pgalloc.h                 |  88 +++++-----
 include/asm-generic/tlb.h                     |  11 ++
 include/linux/mm.h                            | 151 +++++++++++++-----
 include/linux/mm_types.h                      |  18 ---
 include/linux/page-flags.h                    |  30 +++-
 include/linux/pgtable.h                       |  80 ++++++++++
 mm/memory.c                                   |   8 +-
 38 files changed, 585 insertions(+), 384 deletions(-)

-- 
2.40.1

On Mon, 24 Jul 2023, Vishal Moola (Oracle) wrote:

> The MM subsystem is trying to shrink struct page. This patchset
> introduces a memory descriptor for page table tracking - struct ptdesc.
> 
> This patchset introduces ptdesc, splits ptdesc from struct page, and
> converts many callers of page table constructor/destructors to use ptdescs.
> 
> Ptdesc is a foundation to further standardize page tables, and eventually
> allow for dynamic allocation of page tables independent of struct page.
> However, the use of pages for page table tracking is quite deeply
> ingrained and varied across archictectures, so there is still a lot of
> work to be done before that can happen.

Others may differ, but it remains the case that I see no point to this
patchset, until the minimal descriptor that replaces struct page is
working, and struct page then becomes just overhead.  Until that time,
let architectures continue to use struct page as they do - whyever not?

Hugh

> 
> This is rebased on mm-unstable.
> 
> v7:
>   Drop s390 gmap ptdesc conversions - gmap is unecessary complication
>     that can be dealt with later
>   Be more thorough with ptdesc struct sanity checks and comments
>   Rebase onto mm-unstable
> 
> Vishal Moola (Oracle) (31):
>   mm: Add PAGE_TYPE_OP folio functions
>   pgtable: Create struct ptdesc
>   mm: add utility functions for ptdesc
>   mm: Convert pmd_pgtable_page() callers to use pmd_ptdesc()
>   mm: Convert ptlock_alloc() to use ptdescs
>   mm: Convert ptlock_ptr() to use ptdescs
>   mm: Convert pmd_ptlock_init() to use ptdescs
>   mm: Convert ptlock_init() to use ptdescs
>   mm: Convert pmd_ptlock_free() to use ptdescs
>   mm: Convert ptlock_free() to use ptdescs
>   mm: Create ptdesc equivalents for pgtable_{pte,pmd}_page_{ctor,dtor}
>   powerpc: Convert various functions to use ptdescs
>   x86: Convert various functions to use ptdescs
>   s390: Convert various pgalloc functions to use ptdescs
>   mm: Remove page table members from struct page
>   pgalloc: Convert various functions to use ptdescs
>   arm: Convert various functions to use ptdescs
>   arm64: Convert various functions to use ptdescs
>   csky: Convert __pte_free_tlb() to use ptdescs
>   hexagon: Convert __pte_free_tlb() to use ptdescs
>   loongarch: Convert various functions to use ptdescs
>   m68k: Convert various functions to use ptdescs
>   mips: Convert various functions to use ptdescs
>   nios2: Convert __pte_free_tlb() to use ptdescs
>   openrisc: Convert __pte_free_tlb() to use ptdescs
>   riscv: Convert alloc_{pmd, pte}_late() to use ptdescs
>   sh: Convert pte_free_tlb() to use ptdescs
>   sparc64: Convert various functions to use ptdescs
>   sparc: Convert pgtable_pte_page_{ctor, dtor}() to ptdesc equivalents
>   um: Convert {pmd, pte}_free_tlb() to use ptdescs
>   mm: Remove pgtable_{pmd, pte}_page_{ctor, dtor}() wrappers
> 
>  Documentation/mm/split_page_table_lock.rst    |  12 +-
>  .../zh_CN/mm/split_page_table_lock.rst        |  14 +-
>  arch/arm/include/asm/tlb.h                    |  12 +-
>  arch/arm/mm/mmu.c                             |   7 +-
>  arch/arm64/include/asm/tlb.h                  |  14 +-
>  arch/arm64/mm/mmu.c                           |   7 +-
>  arch/csky/include/asm/pgalloc.h               |   4 +-
>  arch/hexagon/include/asm/pgalloc.h            |   8 +-
>  arch/loongarch/include/asm/pgalloc.h          |  27 ++--
>  arch/loongarch/mm/pgtable.c                   |   7 +-
>  arch/m68k/include/asm/mcf_pgalloc.h           |  47 +++---
>  arch/m68k/include/asm/sun3_pgalloc.h          |   8 +-
>  arch/m68k/mm/motorola.c                       |   4 +-
>  arch/mips/include/asm/pgalloc.h               |  32 ++--
>  arch/mips/mm/pgtable.c                        |   8 +-
>  arch/nios2/include/asm/pgalloc.h              |   8 +-
>  arch/openrisc/include/asm/pgalloc.h           |   8 +-
>  arch/powerpc/mm/book3s64/mmu_context.c        |  10 +-
>  arch/powerpc/mm/book3s64/pgtable.c            |  32 ++--
>  arch/powerpc/mm/pgtable-frag.c                |  56 +++----
>  arch/riscv/include/asm/pgalloc.h              |   8 +-
>  arch/riscv/mm/init.c                          |  16 +-
>  arch/s390/include/asm/pgalloc.h               |   4 +-
>  arch/s390/include/asm/tlb.h                   |   4 +-
>  arch/s390/mm/pgalloc.c                        | 128 +++++++--------
>  arch/sh/include/asm/pgalloc.h                 |   9 +-
>  arch/sparc/mm/init_64.c                       |  17 +-
>  arch/sparc/mm/srmmu.c                         |   5 +-
>  arch/um/include/asm/pgalloc.h                 |  18 +--
>  arch/x86/mm/pgtable.c                         |  47 +++---
>  arch/x86/xen/mmu_pv.c                         |   2 +-
>  include/asm-generic/pgalloc.h                 |  88 +++++-----
>  include/asm-generic/tlb.h                     |  11 ++
>  include/linux/mm.h                            | 151 +++++++++++++-----
>  include/linux/mm_types.h                      |  18 ---
>  include/linux/page-flags.h                    |  30 +++-
>  include/linux/pgtable.h                       |  80 ++++++++++
>  mm/memory.c                                   |   8 +-
>  38 files changed, 585 insertions(+), 384 deletions(-)
> 
> -- 
> 2.40.1

On Mon, Jul 24, 2023 at 09:41:36PM -0700, Hugh Dickins wrote:
> On Mon, 24 Jul 2023, Vishal Moola (Oracle) wrote:
> 
> > The MM subsystem is trying to shrink struct page. This patchset
> > introduces a memory descriptor for page table tracking - struct ptdesc.
> > 
> > This patchset introduces ptdesc, splits ptdesc from struct page, and
> > converts many callers of page table constructor/destructors to use ptdescs.
> > 
> > Ptdesc is a foundation to further standardize page tables, and eventually
> > allow for dynamic allocation of page tables independent of struct page.
> > However, the use of pages for page table tracking is quite deeply
> > ingrained and varied across archictectures, so there is still a lot of
> > work to be done before that can happen.
> 
> Others may differ, but it remains the case that I see no point to this
> patchset, until the minimal descriptor that replaces struct page is
> working, and struct page then becomes just overhead.  Until that time,
> let architectures continue to use struct page as they do - whyever not?

Because it's easier for architecture maintainers to understand what they
should and shouldn't be using.  Look at the definition:

+struct ptdesc {
+	unsigned long __page_flags;
+
+	union {
+		struct rcu_head pt_rcu_head;
+		struct list_head pt_list;
+		struct {
+			unsigned long _pt_pad_1;
+			pgtable_t pmd_huge_pte;
+		};
+	};
+	unsigned long __page_mapping;
+
+	union {
+		struct mm_struct *pt_mm;
+		atomic_t pt_frag_refcount;
+	};
+
+	union {
+		unsigned long _pt_pad_2;
+#if ALLOC_SPLIT_PTLOCKS
+		spinlock_t *ptl;
+#else
+		spinlock_t ptl;
+#endif
+	};
+	unsigned int __page_type;
+	atomic_t _refcount;
+#ifdef CONFIG_MEMCG
+	unsigned long pt_memcg_data;
+#endif
+};

It's still a 31-line struct definition, I'll grant you.  But it's far
easier to comprehend than the definition of struct page (~140 lines).
An architecture maintainer can look at it and see what might be available,
and what is already used.  And hopefully we'll have less "Oh, I'll just
use page->private".  It's really not fair to expect arch maintainers to
learn so much about the mm.

It's still messier than I would like, but I don't think we can do better
for now.  I don't understand why you're so interested in delaying doing
this work.