/* Page allocator. Hands out memory in page-size chunks.
See malloc.h for an allocator that hands out smaller
- chunks. */
+ chunks.
+
+ We simply use a linked list of free pages. It would be
+ straightforward to add all available memory to this free list
+ at initialization time. In practice, though, that's really
+ slow because it causes the emulator we're running under to
+ have to fault in every page of memory. So instead we only add
+ pages to the free list as needed. */
/* A free page owned by the page allocator. */
struct page
list_elem free_elem; /* Free list element. */
};
+/* Keeps multiple threads away from free_pages and
+ uninit_start. */
static struct lock lock;
+
+/* List of free pages. */
static struct list free_pages;
+
+/* Range of pages (expressed as byte pointers to the beginnings
+ of pages) that we haven't added to the free list yet. */
static uint8_t *uninit_start, *uninit_end;
+/* Initializes the page allocator. */
void
palloc_init (void)
{
+ extern char _start, _end;
+
/* Kernel static code and data, in 4 kB pages.
-
We can figure this out because the linker records the start
and end of the kernel as _start and _end. See
- kernel.lds. */
- extern char _start, _end;
+ kernel.lds.S. */
size_t kernel_pages = (&_end - &_start + 4095) / 4096;
/* Then we know how much is available to allocate. */
list_init (&free_pages);
}
+/* Obtains and returns a free page. If PAL_ZERO is set in FLAGS,
+ then the page is filled with zeros. If no pages are
+ available, returns a null pointer, unless PAL_ASSERT is set in
+ FLAGS, in which case the kernel panics. */
void *
palloc_get (enum palloc_flags flags)
{
lock_acquire (&lock);
+ /* If there's a page in the free list, take it.
+ Otherwise, if there's a page not yet added to the free list,
+ use it.
+ Otherwise, we're out of memory. */
if (!list_empty (&free_pages))
page = list_entry (list_pop_front (&free_pages), struct page, free_elem);
else if (uninit_start < uninit_end)
return page;
}
+/* Frees PAGE. */
void
palloc_free (void *page_)
{