1 #include "threads/palloc.h"
10 #include "threads/loader.h"
11 #include "threads/synch.h"
12 #include "threads/vaddr.h"
14 /* Page allocator. Hands out memory in page-size (or
15 page-multiple) chunks. See malloc.h for an allocator that
16 hands out smaller chunks.
18 System memory is divided into two "pools" called the kernel
19 and user pools. The user pool is for user (virtual) memory
20 pages, the kernel pool for everything else. The idea here is
21 that the kernel needs to have memory for its own operations
22 even if user processes are swapping like mad.
24 By default, half of system RAM is given to the kernel pool and
25 half to the user pool. That should be huge overkill for the
26 kernel pool, but that's just fine for demonstration purposes. */
31 struct lock lock; /* Mutual exclusion. */
32 struct bitmap *used_map; /* Bitmap of free pages. */
33 uint8_t *base; /* Base of pool. */
36 /* Two pools: one for kernel data, one for user pages. */
37 struct pool kernel_pool, user_pool;
39 /* Maximum number of pages to put in user pool. */
40 size_t user_page_limit = SIZE_MAX;
42 static void init_pool (struct pool *, void *base, size_t page_cnt,
44 static bool page_from_pool (const struct pool *, void *page);
46 /* Initializes the page allocator. */
50 /* Free memory starts at 1 MB and runs to the end of RAM. */
51 uint8_t *free_start = ptov (1024 * 1024);
52 uint8_t *free_end = ptov (ram_pages * PGSIZE);
53 size_t free_pages = (free_end - free_start) / PGSIZE;
54 size_t user_pages = free_pages / 2;
56 if (user_pages > user_page_limit)
57 user_pages = user_page_limit;
58 kernel_pages = free_pages - user_pages;
60 /* Give half of memory to kernel, half to user. */
61 init_pool (&kernel_pool, free_start, kernel_pages, "kernel pool");
62 init_pool (&user_pool, free_start + kernel_pages * PGSIZE,
63 user_pages, "user pool");
66 /* Obtains and returns a group of PAGE_CNT contiguous free pages.
67 If PAL_USER is set, the pages are obtained from the user pool,
68 otherwise from the kernel pool. If PAL_ZERO is set in FLAGS,
69 then the pages are filled with zeros. If too few pages are
70 available, returns a null pointer, unless PAL_ASSERT is set in
71 FLAGS, in which case the kernel panics. */
73 palloc_get_multiple (enum palloc_flags flags, size_t page_cnt)
75 struct pool *pool = flags & PAL_USER ? &user_pool : &kernel_pool;
82 lock_acquire (&pool->lock);
83 page_idx = bitmap_scan_and_flip (pool->used_map, 0, page_cnt, false);
84 lock_release (&pool->lock);
86 if (page_idx != BITMAP_ERROR)
87 pages = pool->base + PGSIZE * page_idx;
94 memset (pages, 0, PGSIZE * page_cnt);
98 if (flags & PAL_ASSERT)
99 PANIC ("palloc_get: out of pages");
105 /* Obtains a single free page and returns its kernel virtual
107 If PAL_USER is set, the page is obtained from the user pool,
108 otherwise from the kernel pool. If PAL_ZERO is set in FLAGS,
109 then the page is filled with zeros. If no pages are
110 available, returns a null pointer, unless PAL_ASSERT is set in
111 FLAGS, in which case the kernel panics. */
113 palloc_get_page (enum palloc_flags flags)
115 return palloc_get_multiple (flags, 1);
118 /* Frees the PAGE_CNT pages starting at PAGES. */
120 palloc_free_multiple (void *pages, size_t page_cnt)
125 ASSERT (pg_ofs (pages) == 0);
126 if (pages == NULL || page_cnt == 0)
129 if (page_from_pool (&kernel_pool, pages))
131 else if (page_from_pool (&user_pool, pages))
136 page_idx = pg_no (pages) - pg_no (pool->base);
139 memset (pages, 0xcc, PGSIZE * page_cnt);
142 ASSERT (bitmap_all (pool->used_map, page_idx, page_cnt));
143 bitmap_set_multiple (pool->used_map, page_idx, page_cnt, false);
146 /* Frees the page at PAGE. */
148 palloc_free_page (void *page)
150 palloc_free_multiple (page, 1);
153 /* Initializes pool P as starting at START and ending at END,
154 naming it NAME for debugging purposes. */
156 init_pool (struct pool *p, void *base, size_t page_cnt, const char *name)
158 /* We'll put the pool's used_map at its base.
159 Calculate the space needed for the bitmap
160 and subtract it from the pool's size. */
161 size_t bm_pages = DIV_ROUND_UP (bitmap_buf_size (page_cnt), PGSIZE);
162 if (bm_pages > page_cnt)
163 PANIC ("Not enough memory in %s for bitmap.", name);
164 page_cnt -= bm_pages;
166 printf ("%zu pages available in %s.\n", page_cnt, name);
168 /* Initialize the pool. */
169 lock_init (&p->lock);
170 p->used_map = bitmap_create_in_buf (page_cnt, base, bm_pages * PGSIZE);
171 p->base = base + bm_pages * PGSIZE;
174 /* Returns true if PAGE was allocated from POOL,
177 page_from_pool (const struct pool *pool, void *page)
179 size_t page_no = pg_no (page);
180 size_t start_page = pg_no (pool->base);
181 size_t end_page = start_page + bitmap_size (pool->used_map);
183 return page_no >= start_page && page_no < end_page;