VMA

This document describes the Virtual Memory Area (VMA) subsystem in the Kyronix kernel. It is the child of Memory Management.

Source

kernel/mm/vma.c, kernel/mm/vma.h

Overview

VMAs track which virtual address ranges are mapped in each address space. They are used for demand paging, mmap/munmap tracking, and page fault handling.

Protection Constants

Constant	Value	Description
`PROT_READ`	0x1	Read permission
`PROT_WRITE`	0x2	Write permission
`PROT_EXEC`	0x4	Execute permission

Data Structure

typedef struct {
    uint64_t start;         // Start virtual address
    uint64_t end;           // End virtual address
    uint32_t prot;          // Protection flags (PROT_READ/WRITE/EXEC)
    uint32_t map_flags;     // VMM mapping flags
    uint8_t  used;          // Slot in use
    uint8_t  free_on_unmap; // Free physical pages on unmap
} vmm_vma_t;

VMAs are stored in a flat array of 2048 entries (VMM_VMA_MAX) inside each vmm_space_t. All lookups are linear scans.

Functions

Function	Description
`vma_reset(sp)`	Zero all VMA slots
`vma_copy(dst, src)`	Copy VMA array between address spaces
`vma_conflicts(sp, start, len)`	Check for overlapping VMAs
`vma_add(sp, start, len, prot, flags, free_on_unmap)`	Register a new VMA
`vma_remove(sp, start, len)`	Remove a VMA range (with splitting)
`vma_remove_overlaps(sp, start, len)`	Remove all overlapping VMAs
`vma_protect(sp, start, len, prot)`	Change protection on a range
`vma_page_fault_allowed(sp, addr, write, exec)`	Check if a page fault can be handled
`vma_page_flags(sp, addr)`	Convert VMA protection to PTE flags
`vma_range_ok(sp, start, len)`	Validate range is fully covered by VMAs

VMA Splitting

When removing a sub-range from a VMA, four cases apply:

No leftovers: Clear the VMA entirely
Left only: Shrink v->end = start
Right only: Shrink v->start = end
Split: Create two VMAs (left and right of the hole)

Case 4 requires an empty VMA slot and may fail with -ENOMEM.

Page Fault Integration

vma_page_fault_allowed() is called by the VMM’s demand paging path. It returns true only if:

A VMA covers the faulting address
The VMA has free_on_unmap set
The VMA has sufficient permissions (PROT_READ minimum, PROT_WRITE for writes, PROT_EXEC for instruction fetches)

This enables lazy allocation: pages are allocated on first access rather than at mmap time.

Last reviewed: 2026-07-22

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