#include <list.h>
#include <stddef.h>
#include <stdint.h>
+#include <stdio.h>
#include <string.h>
#include "threads/init.h"
#include "threads/loader.h"
See malloc.h for an allocator that hands out smaller
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. */
+ System memory is divided into two "pools" called the kernel
+ and user pools. The user pool is for user (virtual) memory
+ pages, the kernel pool for everything else. The idea here is
+ that the kernel needs to have memory for its own operations
+ even if user processes are swapping like mad.
+
+ By default, half of system RAM is given to the kernel pool and
+ half to the user pool. That should be huge overkill for the
+ kernel pool, but that's just fine for demonstration purposes.
+
+ Within each pool, 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. */
+ list_elem elem; /* Free list element. */
};
-/* Keeps multiple threads away from free_pages and
- uninit_start. */
-static struct lock lock;
+/* A memory pool. */
+struct pool
+ {
+ struct lock lock; /* Mutual exclusion. */
+ uint8_t *start, *end; /* Start and end of pool. */
+ uint8_t *uninit; /* First page not yet in free_list. */
+ struct list free_list; /* Free pages. */
+ };
-/* List of free pages. */
-static struct list free_pages;
+/* Two pools: one for kernel data, one for user pages. */
+struct pool kernel_pool, user_pool;
-/* 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;
+static void init_pool (struct pool *, void *start, void *end,
+ const char *name);
+static bool page_from_pool (const struct pool *, void *page);
/* 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.S. */
- size_t kernel_pages = (&_end - &_start + 4095) / 4096;
-
- /* Then we know how much is available to allocate. */
- uninit_start = ptov (LOADER_KERN_BASE + kernel_pages * PGSIZE);
- uninit_end = ptov (ram_pages * PGSIZE);
-
- /* Initialize other variables. */
- lock_init (&lock, "palloc");
- list_init (&free_pages);
+ /* End of the kernel as recorded by the linker.
+ See kernel.lds.S. */
+ extern char _end;
+
+ /* Free memory. */
+ uint8_t *free_start = pg_round_up (&_end);
+ uint8_t *free_end = ptov (ram_pages * PGSIZE);
+ size_t free_pages = (free_end - free_start) / PGSIZE;
+ uint8_t *free_middle = free_start + free_pages / 2 * PGSIZE;
+
+ /* Give half of memory to kernel, half to user. */
+ init_pool (&kernel_pool, free_start, free_middle, "kernel pool");
+ init_pool (&user_pool, free_middle, free_end, "user pool");
}
/* Obtains and returns a free page. If PAL_ZERO is set in FLAGS,
void *
palloc_get (enum palloc_flags flags)
{
+ struct pool *pool = flags & PAL_USER ? &user_pool : &kernel_pool;
struct page *page;
- lock_acquire (&lock);
+ lock_acquire (&pool->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)
+ if (!list_empty (&pool->free_list))
+ page = list_entry (list_pop_front (&pool->free_list), struct page, elem);
+ else if (pool->uninit < pool->end)
{
- page = (struct page *) uninit_start;
- uninit_start += PGSIZE;
+ page = (struct page *) pool->uninit;
+ pool->uninit += PGSIZE;
}
else
page = NULL;
PANIC ("palloc_get: out of pages");
}
- lock_release (&lock);
+ lock_release (&pool->lock);
return page;
}
void
palloc_free (void *page_)
{
+ struct pool *pool;
struct page *page = page_;
ASSERT (page == pg_round_down (page));
+ if (page_from_pool (&kernel_pool, page))
+ pool = &kernel_pool;
+ else if (page_from_pool (&user_pool, page))
+ pool = &user_pool;
+ else
+ PANIC ("freeing invalid pointer");
+
#ifndef NDEBUG
memset (page, 0xcc, PGSIZE);
#endif
- lock_acquire (&lock);
- list_push_front (&free_pages, &page->free_elem);
- lock_release (&lock);
+ lock_acquire (&pool->lock);
+ list_push_front (&pool->free_list, &page->elem);
+ lock_release (&pool->lock);
+}
+
+/* Initializes pool P as starting at START and ending at END,
+ naming it NAME for debugging purposes. */
+static void
+init_pool (struct pool *p, void *start, void *end, const char *name)
+{
+ ASSERT (pg_ofs (start) == 0);
+ ASSERT (pg_ofs (end) == 0);
+
+ printf ("%d kB allocated for %s.\n",
+ (PGSIZE / 1024) * (pg_no (end) - pg_no (start)), name);
+
+ lock_init (&p->lock, name);
+ p->start = p->uninit = start;
+ p->end = end;
+ list_init (&p->free_list);
+}
+
+/* Returns true if PAGE was allocated from POOL,
+ false otherwise. */
+static bool
+page_from_pool (const struct pool *pool, void *page_)
+{
+ uint8_t *page = page_;
+
+ return page >= pool->start && page < pool->uninit;
}
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