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Buffers and Buffer Heads
When a block is stored in memory (say, after a read or pending a write), it is stored in a buffer . Each buffer is associated with exactly one block. The buffer serves as the object that represents a disk block in memory. Recall that a block comprises one or more sectors, but is no more than a page in size. Therefore, a single page can hold one or more blocks in memory. Because the kernel requires some associated control information to accompany the data (such as from which block device and which specific block the buffer is), each buffer is associated with a descriptor. The descriptor is called a buffer head and is of type struct buffer_head. The buffer_head structure holds all the information that the kernel needs to manipulate buffers and is defined in <linux/buffer_head.h>. Take a look at this structure, with comments describing each field:
struct buffer_head {
unsigned long b_state; /* buffer state flags */
atomic_t b_count; /* buffer usage counter */
struct buffer_head *b_this_page; /* buffers using this page */
struct page *b_page; /* page storing this buffer */
sector_t b_blocknr; /* logical block number */
u32 b_size; /* block size (in bytes) */
char *b_data; /* buffer in the page */
struct block_device *b_bdev; /* device where block resides */
bh_end_io_t *b_end_io; /* I/O completion method */
void *b_private; /* data for completion method */
struct list_head b_assoc_buffers; /* list of associated mappings */
}; The b_state field specifies the state of this particular buffer. It can be one or more of the flags in Table 13.1. The legal flags are stored in the bh_state_bits enumeration, which is defined in <linux/buffer_head.h>.
static inline void get_bh(struct buffer_head *bh)
{
atomic_inc(&bh->b_count);
}
static inline void put_bh(struct buffer_head *bh)
{
atomic_dec(&bh->b_count);
}
Before manipulating a buffer head, you must increment its reference count via get_bh() to ensure that the buffer head is not deallocated out from under you. When finished with the buffer head, decrement the reference count via put_bh().Chapter 15, "The Page Cache and Page Writeback," where the address_space structure and the pdflush daemons are discussed.The second issue with buffer heads is that they describe only a single buffer. When used as the container for all I/O operations, the buffer head forces the kernel to break up potentially large block I/O operations (say, a write) into multiple buffer_head structures. This results in needless overhead and space consumption. As a result, the primary goal of the 2.5 development kernel was to introduce a new, flexible, and lightweight container for block I/O operations. The result is the bio structure, which is discussed in the next section.
