buffer.c

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 * bread() reads a specified block and returns the buffer that contains
 * it. It returns NULL if the block was unreadable.
 */
struct buffer_head * bread(kdev_t dev, int block, int size)
{
	struct buffer_head * bh;

	if (!(bh = getblk(dev, block, size))) {
		printk("VFS: bread: impossible error\n");
		return NULL;
	}
	if (buffer_uptodate(bh))
		return bh;
	ll_rw_block(READ, 1, &bh);
	wait_on_buffer(bh);
	if (buffer_uptodate(bh))
		return bh;
	brelse(bh);
	return NULL;
}

/*
 * Ok, breada can be used as bread, but additionally to mark other
 * blocks for reading as well. End the argument list with a negative
 * number.
 */

#define NBUF 16

struct buffer_head * breada(kdev_t dev, int block, int bufsize,
	unsigned int pos, unsigned int filesize)
{
	struct buffer_head * bhlist[NBUF];
	unsigned int blocks;
	struct buffer_head * bh;
	int index;
	int i, j;

	if (pos >= filesize)
		return NULL;

	if (block < 0 || !(bh = getblk(dev,block,bufsize)))
		return NULL;

	index = BUFSIZE_INDEX(bh->b_size);

	if (buffer_uptodate(bh))
		return(bh);   
	else ll_rw_block(READ, 1, &bh);

	blocks = (filesize - pos) >> (9+index);

	if (blocks < (read_ahead[MAJOR(dev)] >> index))
		blocks = read_ahead[MAJOR(dev)] >> index;
	if (blocks > NBUF) 
		blocks = NBUF;

/*	if (blocks) printk("breada (new) %d blocks\n",blocks); */


	bhlist[0] = bh;
	j = 1;
	for(i=1; i<blocks; i++) {
		bh = getblk(dev,block+i,bufsize);
		if (buffer_uptodate(bh)) {
			brelse(bh);
			break;
		}
		else bhlist[j++] = bh;
	}

	/* Request the read for these buffers, and then release them */
	if (j>1)  
		ll_rw_block(READA, (j-1), bhlist+1); 
	for(i=1; i<j; i++)
		brelse(bhlist[i]);

	/* Wait for this buffer, and then continue on */
	bh = bhlist[0];
	wait_on_buffer(bh);
	if (buffer_uptodate(bh))
		return bh;
	brelse(bh);
	return NULL;
}

static void get_more_buffer_heads(void)
{
	struct wait_queue wait = { current, NULL };
	struct buffer_head * bh;

	while (!unused_list) {
		/*
		 * This is critical.  We can't swap out pages to get
		 * more buffer heads, because the swap-out may need
		 * more buffer-heads itself.  Thus GFP_ATOMIC.
		 *
		 * This is no longer true, it is GFP_BUFFER again, the
		 * swapping code now knows not to perform I/O when that
		 * GFP level is specified... -DaveM
		 */
		/* we now use kmalloc() here instead of gfp as we want
                   to be able to easily release buffer heads - they
                   took up quite a bit of memory (tridge) */
		bh = (struct buffer_head *) kmalloc(sizeof(*bh),GFP_BUFFER);
		if (bh) {
			put_unused_buffer_head(bh);
			nr_buffer_heads++;
			return;
		}

		/*
		 * Uhhuh. We're _really_ low on memory. Now we just
		 * wait for old buffer heads to become free due to
		 * finishing IO..
		 */
		run_task_queue(&tq_disk);

		/*
		 * Set our state for sleeping, then check again for buffer heads.
		 * This ensures we won't miss a wake_up from an interrupt.
		 */
		add_wait_queue(&buffer_wait, &wait);
		current->state = TASK_UNINTERRUPTIBLE;
		if (!unused_list && !reuse_list)
			schedule();
		recover_reusable_buffer_heads();
		remove_wait_queue(&buffer_wait, &wait);
		current->state = TASK_RUNNING;
	}

}

static struct buffer_head * get_unused_buffer_head(void)
{
	struct buffer_head * bh;

	recover_reusable_buffer_heads();
	get_more_buffer_heads();
	if (!unused_list)
		return NULL;
	bh = unused_list;
	unused_list = bh->b_next_free;
	nr_unused_buffer_heads--;
	return bh;
}

/*
 * Create the appropriate buffers when given a page for data area and
 * the size of each buffer.. Use the bh->b_this_page linked list to
 * follow the buffers created.  Return NULL if unable to create more
 * buffers.
 */
static struct buffer_head * create_buffers(unsigned long page, unsigned long size)
{
	struct buffer_head *bh, *head;
	long offset;

	head = NULL;
	offset = PAGE_SIZE;
	while ((offset -= size) >= 0) {
		bh = get_unused_buffer_head();
		if (!bh)
			goto no_grow;

		bh->b_dev = B_FREE;  /* Flag as unused */
		bh->b_this_page = head;
		head = bh;

		bh->b_state = 0;
		bh->b_next_free = NULL;
		bh->b_count = 0;
		bh->b_size = size;

		bh->b_data = (char *) (page+offset);
		bh->b_list = 0;
	}
	return head;
/*
 * In case anything failed, we just free everything we got.
 */
no_grow:
	bh = head;
	while (bh) {
		head = bh;
		bh = bh->b_this_page;
		put_unused_buffer_head(head);
	}
	return NULL;
}

/* Run the hooks that have to be done when a page I/O has completed. */
static inline void after_unlock_page (struct page * page)
{
	if (clear_bit(PG_decr_after, &page->flags))
		atomic_dec(&nr_async_pages);
	if (clear_bit(PG_free_after, &page->flags))
		__free_page(page);
#ifndef NO_MM
	if (clear_bit(PG_swap_unlock_after, &page->flags))
		swap_after_unlock_page(page->swap_unlock_entry);
#endif
}

/*
 * Free all temporary buffers belonging to a page.
 * This needs to be called with interrupts disabled.
 */
static inline void free_async_buffers (struct buffer_head * bh)
{
	struct buffer_head * tmp;

	tmp = bh;
	do {
		if (!test_bit(BH_FreeOnIO, &tmp->b_state)) {
			printk ("Whoops: unlock_buffer: "
				"async IO mismatch on page.\n");
			return;
		}
		tmp->b_next_free = reuse_list;
		reuse_list = tmp;
		clear_bit(BH_FreeOnIO, &tmp->b_state);
		tmp = tmp->b_this_page;
	} while (tmp != bh);
}

/*
 * Start I/O on a page.
 * This function expects the page to be locked and may return before I/O is complete.
 * You then have to check page->locked, page->uptodate, and maybe wait on page->wait.
 */
int brw_page(int rw, struct page *page, kdev_t dev, int b[], int size, int bmap)
{
	struct buffer_head *bh, *prev, *next, *arr[MAX_BUF_PER_PAGE];
	int block, nr;

	if (!PageLocked(page))
		panic("brw_page: page not locked for I/O");
	clear_bit(PG_uptodate, &page->flags);
	clear_bit(PG_error, &page->flags);
	/*
	 * Allocate buffer heads pointing to this page, just for I/O.
	 * They do _not_ show up in the buffer hash table!
	 * They are _not_ registered in page->buffers either!
	 */
	bh = create_buffers(page_address(page), size);
	if (!bh) {
		clear_bit(PG_locked, &page->flags);
		wake_up(&page->wait);
		return -ENOMEM;
	}
	nr = 0;
	next = bh;
	do {
		struct buffer_head * tmp;
		block = *(b++);

		set_bit(BH_FreeOnIO, &next->b_state);
		next->b_list = BUF_CLEAN;
		next->b_dev = dev;
		next->b_blocknr = block;
		next->b_count = 1;
		next->b_flushtime = 0;
		set_bit(BH_Uptodate, &next->b_state);

		/*
		 * When we use bmap, we define block zero to represent
		 * a hole.  ll_rw_page, however, may legitimately
		 * access block zero, and we need to distinguish the
		 * two cases.
		 */
		if (bmap && !block) {
			memset(next->b_data, 0, size);
			next->b_count--;
			continue;
		}
		tmp = get_hash_table(dev, block, size);
		if (tmp) {
			if (!buffer_uptodate(tmp)) {
				if (rw == READ)
					ll_rw_block(READ, 1, &tmp);
				wait_on_buffer(tmp);
			}
			if (rw == READ) 
				memcpy(next->b_data, tmp->b_data, size);
			else {
				memcpy(tmp->b_data, next->b_data, size);
				mark_buffer_dirty(tmp, 0);
			}
			brelse(tmp);
			next->b_count--;
			continue;
		}
		if (rw == READ)
			clear_bit(BH_Uptodate, &next->b_state);
		else
			set_bit(BH_Dirty, &next->b_state);
		arr[nr++] = next;
	} while (prev = next, (next = next->b_this_page) != NULL);
	prev->b_this_page = bh;

	if (nr) {
		ll_rw_block(rw, nr, arr);
		/* The rest of the work is done in mark_buffer_uptodate()
		 * and unlock_buffer(). */
	} else {
		unsigned long flags;
		save_flags(flags);
		cli();
		free_async_buffers(bh);
		restore_flags(flags);
		clear_bit(PG_locked, &page->flags);
		set_bit(PG_uptodate, &page->flags);
		wake_up(&page->wait);
		after_unlock_page(page);
		if (waitqueue_active(&buffer_wait))
			wake_up(&buffer_wait);
	}
	++current->maj_flt;
	return 0;
}

/*
 * This is called by end_request() when I/O has completed.
 */
void mark_buffer_uptodate(struct buffer_head * bh, int on)
{
	if (on) {
		struct buffer_head *tmp = bh;
		set_bit(BH_Uptodate, &bh->b_state);
		/* If a page has buffers and all these buffers are uptodate,
		 * then the page is uptodate. */
		do {
			if (!test_bit(BH_Uptodate, &tmp->b_state))
				return;
			tmp=tmp->b_this_page;
		} while (tmp && tmp != bh);
		set_bit(PG_uptodate, &mem_map[MAP_NR(bh->b_data)].flags);
		return;
	}
	clear_bit(BH_Uptodate, &bh->b_state);
}

/*
 * This is called by end_request() when I/O has completed.
 */
void unlock_buffer(struct buffer_head * bh)
{
	unsigned long flags;
	struct buffer_head *tmp;
	struct page *page;

	clear_bit(BH_Lock, &bh->b_state);
	wake_up(&bh->b_wait);
	if (waitqueue_active(&buffer_wait))
		wake_up(&buffer_wait);

	if (!test_bit(BH_FreeOnIO, &bh->b_state))
		return;
	/* This is a temporary buffer used for page I/O. */
	page = mem_map + MAP_NR(bh->b_data);
	if (!PageLocked(page))
		goto not_locked;
	if (bh->b_count != 1)
		goto bad_count;

	if (!test_bit(BH_Uptodate, &bh->b_state))
		set_bit(PG_error, &page->flags);

	/*
	 * Be _very_ careful from here on. Bad things can happen if
	 * two buffer heads end IO at almost the same time and both
	 * decide that the page is now completely done.
	 *
	 * Async buffer_heads are here only as labels for IO, and get
	 * thrown away once the IO for this page is complete.  IO is
	 * deemed complete once all buffers have been visited
	 * (b_count==0) and are now unlocked. We must make sure that
	 * only the _last_ buffer that decrements its count is the one
	 * that free's the page..
	 */
	save_flags(flags);
	cli();
	bh->b_count--;
	tmp = bh;
	do {
		if (tmp->b_count)
			goto still_busy;
		tmp = tmp->b_this_page;
	} while (tmp != bh);

	/* OK, the async IO on this page is complete. */
	free_async_buffers(bh);
	restore_flags(flags);
	clear_bit(PG_locked, &page->flags);
	wake_up(&page->wait);
	after_unlock_page(page);
	wake_up(&buffer_wait);
	return;

still_busy:
	restore_flags(flags);
	return;

not_locked:
	printk ("Whoops: unlock_buffer: async io complete on unlocked page\n");
	return;

bad_count:
	printk ("Whoops: unlock_buffer: b_count != 1 on async io.\n");
	return;
}

/*
 * Generic "readpage" function for block devices that have the normal
 * bmap functionality. This is most of the block device filesystems.
 * Reads the page asynchronously --- the unlock_buffer() and
 * mark_buffer_uptodate() functions propagate buffer state into the
 * page struct once IO has completed.
 */
int generic_readpage(struct inode * inode, struct page * page)
{
	unsigned long block;
	int *p, nr[PAGE_SIZE/512];
	int i;

	page->count++;
	set_bit(PG_locked, &page->flags);
	set_bit(PG_free_after, &page->flags);

	i = PAGE_SIZE >> inode->i_sb->s_blocksize_bits;
	block = page->offset >> inode->i_sb->s_blocksize_bits;
	p = nr;
	do {
		*p = inode->i_op->bmap(inode, block);
		i--;
		block++;
		p++;
	} while (i > 0);

	/* IO start */
	brw_page(READ, page, inode->i_dev, nr, inode->i_sb->s_blocksize, 1);
	return 0;
}

/*
 * Try to increase the number of buffers available: the size argument
 * is used to determine what kind of buffers we want.
 */
static int grow_buffers(int pri, int size)
{