- DEFINE RAM: Random Access Memory is a linear array of bytes indexed by “Physical Address” from 0 to Size-1 → CPU reads/writes bytes at specific addresses.
- DEFINE PAGE_SIZE: Managing individual bytes is too slow → Hardware groups bytes into fixed “Pages” → Standard size is 4096 bytes (0x1000) → Derived from Axiom A01 (getconf).
- DEFINE PAGE FRAME: A physical page of RAM → Starts at Address A where A % 4096 == 0.
- DEFINE PFN: Page Frame Number → Index of the page in the RAM array → Formula: PFN = Physical Address / PAGE_SIZE.
- DEFINE ZONE: Physical RAM is split into ranges called “Zones” due to hardware limits (e.g. DMA < 16MB) → Zone is a range of PFNs [Start, End).
- DEFINE ORDER: Allocator manages blocks of 2^N contiguous pages → “Order N” means a block of size 2^N pages.
- DEFINE BUDDY INFO: Kernel tracks free blocks of each Order per Zone →
/proc/buddyinfo lists these counts.
- AXIOM REAL DATA RAM: Your machine has MemTotal 15776272 kB (Ax: /proc/meminfo) → Bytes = 16,154,902,528 → Pages = 16,154,902,528 / 4096 = 3,944,068 Total Pages.
- AXIOM REAL DATA ZONE: Your machine has “Node 0 Zone DMA32” (Ax: /proc/buddyinfo) → DMA32 is zone for 32-bit addressable RAM (< 4GB).
- AXIOM REAL DATA ORDER-0: BuddyInfo shows DMA32 has 24581 blocks at Order 0 → 24581 Free Single Pages available in this zone.
- DEFINE STRUCT PAGE: Kernel needs metadata (flags, count) for every physical page → “struct page” is 64-byte object (Ax: Sizeof).
- DEFINE VMEMMAP: Array of
struct page for all PFNs → vmemmap[PFN] stores metadata for Physical Page PFN.
- DERIVE FORMULA PFN-TO-PAGE: Address of struct page
p = VMEMMAP_BASE + (PFN × 64).
- DERIVE FORMULA PAGE-TO-PFN: PFN = (Address of
p - VMEMMAP_BASE) / 64.
- ACTION ALLOCATE PAGE ALLOC: Call
alloc_page(GFP_KERNEL) → Kernel finds a free Order-0 block (e.g. one of the 24581 in DMA32) → Returns pointer p.
- TRACE POINTER CALCULATION: Assume
p returned is 0xFFFFEA0000040000 (Axiomatic Example) → Calculate Offset = 0xFFFFEA0000040000 - 0xFFFFEA0000000000 = 0x40000 (262144 bytes).
- TRACE PFN CALCULATION: PFN = Offset / 64 = 262144 / 64 = 4096.
- TRACE PHYS CALCULATION: Phys = PFN × 4096 = 4096 × 4096 = 16,777,216 (0x1000000) → 16MB boundary (Start of DMA32).
- VERIFY ZONE: 16MB is start of DMA32 Zone → Matches “Zone DMA32” from BuddyInfo Axiom ✓.
- DEFINE REFCOUNT:
_refcount is atomic counter at offset 52 in struct page → Tracks usage.
- TRACE REFCOUNT ALLOC:
alloc_page sets _refcount = 1 → Means “Allocated”.
- TRACE REFCOUNT GET:
get_page adds 1 → _refcount = 2.
- TRACE REFCOUNT PUT:
put_page subtracts 1 → _refcount = 1 → Not zero ∴ Page not freed.
- CODE TASK: Implement
page_labs.c to Allocate, Print PFN (using derived formula), Print Phys, Get/Put, and Free.