STRUCT PTDESC(9)

✨ NAME ✨

okay so bestie, this is struct ptdesc(9) from the Kernel API Manual — literally the Memory descriptor for page tables. That’s the whole vibe. 💅

🧚 SYNOPSIS 🧚

Here’s the full struct, exactly as it appears, because we do not mess with C source in this group chat:

struct ptdesc {
    memdesc_flags_t pt_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 {
      pgoff_t pt_index;
      struct mm_struct *pt_mm;
      atomic_t pt_frag_refcount;
  #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING;
      atomic_t pt_share_count;
  #endif;
    };
    union {
      unsigned long _pt_pad_2;
  #if ALLOC_SPLIT_PTLOCKS;
      spinlock_t *ptl;
  #else;
      spinlock_t ptl;
  #endif;
    };
    unsigned int __page_type;
    atomic_t __page_refcount;
  #ifdef CONFIG_MEMCG;
    unsigned long pt_memcg_data;
  #endif;
 };

👯 Members 👯

okay, meet the cast — every single member, no skips, no “we’ll come back to it later” energy:

• pt_flags — enum pt_flags plus zone/node/section.
• {unnamed_union} — anonymous.
• pt_rcu_head — For freeing page table pages.
• pt_list — List of used page tables. Used for s390 gmap shadow pages (which are not linked into the user page tables) and x86 pgds.
• {unnamed_struct} — anonymous.
• _pt_pad_1 — Padding that aliases with page's compound head.
• pmd_huge_pte — Protected by ptdesc->ptl, used for THPs.
• __page_mapping — Aliases with page->mapping. Unused for page tables.
• {unnamed_union} — anonymous.
• pt_index — Used for s390 gmap.
• pt_mm — Used for x86 pgds.
• pt_frag_refcount — For fragmented page table tracking. Powerpc only.
• pt_share_count — Used for HugeTLB PMD page table share count.
• {unnamed_union} — anonymous.
• _pt_pad_2 — Padding to ensure proper alignment.
• ptl — Lock for the page table.
• ptl — Lock for the page table.
• __page_type — Same as page->page_type. Unused for page tables.
• __page_refcount — Same as page refcount.
• pt_memcg_data — Memcg data. Tracked for page tables here.

💖 Description 💖

This struct overlays struct page for now. Do not modify without a good understanding of the issues.

bestie, that is a real warning — do not just roll up and start changing things unless you actually understand the issues. like, fr fr. 💀

🔗 SEE ALSO 🔗

Kernel file ./include/linux/mm_types.h — also go peek at softleaf_t(9), struct folio(9), vm_fault_t(9), enum vm_fault_reason(9), and enum fault_flag(9). iykyk 🦋

✨ cute little sign-off: source/version ./include/linux, date October 2026, page struct ptdesc(9) — bye bestie, slay responsibly 💖🎀