SchOS/kernel/core/kalloc.c

// Physical memory allocator, intended to allocate
// memory for user processes, kernel stacks, page table pages,
// and pipe buffers. Allocates 4096-byte pages.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "spinlock.h"

void freerange(void *vstart, void *vend);
extern char end[]; // first address after kernel loaded from ELF file


struct run {
  struct run *next;
};

struct {
  struct spinlock lock;
  int use_lock;
  struct run *freelist;
} kmem;

// Catch kmem.freelist corruption at the point it's touched, rather than
// letting a bad pointer be silently returned by kalloc() and crash whatever
// code dereferences it minutes later on an unrelated call stack.
static void
check_run(struct run *r, char *where)
{
  if(r == 0)
    return;
  if((addr_t)r % PGSIZE || (char*)r < end || V2P(r) >= PHYSTOP){
    cprintf("kalloc: corrupt freelist entry %p at %s\n", r, where);
    panic("kalloc: corrupt freelist");
  }
}

// vstart..vend is the window the kernel is willing to give away: from the end
// of its own image to the bottom of the DMA arena. Inside that window the
// firmware's map decides what is real -- free each usable region clipped to the
// window, rather than walking the whole span and handing out the holes.
//
// The old code walked it as one range, which is what a BIOS map looks like.
// Under UEFI it is not: OVMF puts an ACPI NVS region at 8 MB, so the run
// containing the kernel stopped there and everything above it was lost. The
// window still bounds things at both ends, so nothing below the kernel image
// and nothing inside the DMA arena is ever freed, whatever the map says.
void
kinit1(void *vstart, void *vend)
{
  initlock(&kmem.lock, "kmem");
  kmem.use_lock = 0;
  kmem.freelist = 0; // empty

  // No map at all means e820_init() could not parse one; it leaves a single
  // synthetic region behind for exactly this reason, so this is belt and
  // braces rather than a path anything takes.
  if(nmem_usable == 0){
    freerange(vstart, vend);
    return;
  }

  addr_t lo_limit = V2P((addr_t)vstart);
  addr_t hi_limit = V2P((addr_t)vend);

  for(int i = 0; i < nmem_usable; i++){
    addr_t lo = mem_usable[i].base;
    addr_t hi = mem_usable[i].end;

    if(lo < lo_limit)
      lo = lo_limit;
    if(hi > hi_limit)
      hi = hi_limit;
    if(lo >= hi)
      continue;

    freerange(P2V(lo), P2V(hi));
  }
}

void
kinit2()
{
  kmem.use_lock = 1;
}

void
freerange(void *vstart, void *vend)
{
  char *p;
  p = (char*)PGROUNDUP((addr_t)vstart);
  for(; p + PGSIZE <= (char*)vend; p += PGSIZE)
    kfree(p);
}

// Free the page of physical memory pointed at by v,
// which normally should have been returned by a
// call to kalloc().  (The exception is when
// initializing the allocator; see kinit above.)
void
kfree(char *v)
{
  struct run *r;

  if((addr_t)v % PGSIZE || v < end || V2P(v) >= PHYSTOP)
    panic("kfree");

  // Fill with junk to catch dangling refs.
  memset(v, 1, PGSIZE);

  if(kmem.use_lock)
    acquire(&kmem.lock);
  check_run(kmem.freelist, "kfree entry");
  r = (struct run*)v;
  r->next = kmem.freelist;
  kmem.freelist = r;
  if(kmem.use_lock)
    release(&kmem.lock);
}

// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
char*
kalloc(void)
{
  struct run *r;

  if(kmem.use_lock)
    acquire(&kmem.lock);
  check_run(kmem.freelist, "kalloc entry");
  r = kmem.freelist;
  if(r)
    kmem.freelist = r->next;

  if(kmem.use_lock)
    release(&kmem.lock);
  return (char*)r;
}

// Total free physical memory, in bytes. Walks the free list; only /proc
// readers call it, so the O(n) scan is not on any hot path.
uint64
kalloc_free_bytes(void)
{
  struct run *r;
  uint64 n = 0;

  if(kmem.use_lock)
    acquire(&kmem.lock);
  for(r = kmem.freelist; r; r = r->next)
    n += PGSIZE;
  if(kmem.use_lock)
    release(&kmem.lock);
  return n;
}