在Linux内核中,并非总使用基于页的方法来承担缓存的任务。内核的早期版本只包含了块缓存,来加速文件操作和提高系统性能。这是来自于其他具有相同结构的类UNIX操作系统的遗产。来自于底层块设备的块缓存在内存的缓冲区中,可以加速读写操作。
与内存页相比,块不仅比较小(大多数情况下),而且长度可变的,依赖于使用的块设备(或文件系统)。随着日渐倾向于使用基于页操作实现的通用文件存取方法,块缓存作为中枢系统缓存的重要性已经逐渐失去。主要的缓存任务现在由页缓存承担。另外,基于块的I/O的标准数据结构,现在已经不再是缓冲区,而是struct bio结构。
缓冲区用作小型的数据传输,一般设计的数据量是与块长度可比拟的。文件系统在处理元数据时,通常会使用此类方法。而裸数据的传输则按页进行,而缓冲区的实现也基于也缓存。
块缓存在结构上由两个部分组成:
1) 缓冲头(buffer head)包含了与缓冲区状态相关的所有管理数据,包括快号、块长度、访问计数器等。这些数据不是直接存储在缓冲头之后,而是存储在物理内存的一个独立区域中,由缓冲头结构中的一个对应的指针表示。
2) 有用数据保存在专门分配的页中,这些页也可能同时存在于页缓存中。这进一步细分了页缓存,如下图所示,在我们的例子中,页划分为4个长度相同的部分,每一部分由其自身的缓冲头描述。缓冲头存储的内存区域与有用数据存储的区域是有关的。
这使得页面可以细分为更小的部分,各顾各部分之间完全连续的(因为缓冲区数据和缓冲头数据是分离的)。因为一个缓冲区由至少512字节组成,每页最多可包括MAX_BUF_PER_PAGE个缓冲区。该常数定义为页面长度的函数。
如果修改了某个缓冲区,则会立即印象到页面的内容(反之也是),因而两个缓存不需要显示同步,毕竟二者的数据是共享的。
当然,有些应用程序在访问块设备时,使用的是块而不是页面,读取文件系统的操作几块,就是一个例子。一个独立的块缓存用于加速此类访问。该块缓存的运作独立于页面缓存,而不是在其上建立的。为此,缓冲头数据结构(对于块缓存和页面缓存是相同的)群聚在一个长度恒定的数组中,各个数组项按LUR方式管理。在一个三个数组项用过之后,将其置于索引位置0,其他数组项相应下移。这意味这最常使用的数组项位于数组的开头,而不常用的数组项将被后退,如果很长时间不使用,则会“掉出”数组。
因为数组的长度,或者说LUR列表中的项数,是一个固定值,在内核运行期间不改变,内核无需运行独立的线程来将缓存长度修正为合理值。相反,内核只需要在一项“掉出”数组时,将相关的缓冲区从缓存删除,以释放内存,用于其他目地。
块缓存实现
块患处不仅仅用作页面缓存的附加功能,对以块而不是页面进行处理的对象来说,块缓存是一个独立的缓存。
数据结构
块缓冲区头
- struct buffer_head {
- unsigned long b_state; /* buffer state bitmap (see above) */
- struct buffer_head *b_this_page;/* circular list of page’s buffers */
- struct page *b_page; /* the page this bh is mapped to */
- sector_t b_blocknr; /* start block number */
- size_t b_size; /* size of mapping */
- char *b_data; /* pointer to data within the page */
- struct block_device *b_bdev;
- bh_end_io_t *b_end_io; /* I/O completion */
- void *b_private; /* reserved for b_end_io */
- struct list_head b_assoc_buffers; /* associated with another mapping */
- struct address_space *b_assoc_map; /* mapping this buffer is
- associated with */
- atomic_t b_count; /* users using this buffer_head */
- };
操作
内核必须提供一组操作,使得其余代码能够轻松有效地利用缓冲区的功能。切记:这些机制对内存中实际缓存的数据没有贡献。
在使用缓冲区之前,内核首先必须创建一个buffer_head结构实例,而其余的函数则对该结构进行操作。因为创建新缓冲头是一个频繁重现的任务,他应该尽快执行。这是一种很经典的情形,可使用slab缓存解决。
切记:内核源代码确实提供了一些函数,可用作前端,来创建和销毁缓冲头。alloc_buffer_head生成一个新的缓冲头,而free_buffer_head销毁一个显存的缓冲头。
- /*分配buffer_head*/
- struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
- {
- /*从slab中分配空间*/
- struct buffer_head *ret = kmem_cache_alloc(bh_cachep, gfp_flags);
- if (ret) {
- /*初始化*/
- INIT_LIST_HEAD(&ret->b_assoc_buffers);
- get_cpu_var(bh_accounting).nr++;
- recalc_bh_state();
- put_cpu_var(bh_accounting);
- }
- return ret;
- }
页缓存和块缓存的交互
一页划分为几个数据单元,但缓冲头保存在独立的内存区中,与实际数据无关。与缓冲区的交互没有改变的页的内容,缓冲区只不过为页的数据提供了一个新的视图。
为支持页与缓冲区的交互,需要使用struct page的private成员。其类型为unsigned long,可用作指向虚拟地址空间中任何位置的指针。
Private成员还可以用作其他用途,根据页的具体用途,可能与缓冲头完全无关。但其主要的用途是关联缓冲区和页。这样的话,private指向将页划分为更小单位的第一个缓冲头。各个缓冲头通过b_this_page链接为一个环形链表。在该链表中每个缓冲头的b_this_page成员指向下一个缓冲头,而最后一个缓冲头的b_this_page成员指向第一个缓冲头。这使得内核从page结构开始,可以轻易地扫描与页关联的所有buffer_head实例。
内核提供cteate_empty_buffers函数关联page和buffer_head结构之间的关联:
- /*
- * We attach and possibly dirty the buffers atomically wrt
- * __set_page_dirty_buffers() via private_lock. try_to_free_buffers
- * is already excluded via the page lock.
- */
- void create_empty_buffers(struct page *page,
- unsigned long blocksize, unsigned long b_state)
- {
- struct buffer_head *bh, *head, *tail;
- head = alloc_page_buffers(page, blocksize, 1);
- bh = head;
- /*遍历所有缓冲头,设置其状态,并建立一个环形链表*/
- do {
- bh->b_state |= b_state;
- tail = bh;
- bh = bh->b_this_page;
- } while (bh);
- tail->b_this_page = head;
- spin_lock(&page->mapping->private_lock);
- /*缓冲区的状态依赖于内存页面中数据的状态*/
- if (PageUptodate(page) || PageDirty(page)) {
- bh = head;
- do {/*设置相关标志*/
- if (PageDirty(page))
- set_buffer_dirty(bh);
- if (PageUptodate(page))
- set_buffer_uptodate(bh);
- bh = bh->b_this_page;
- } while (bh != head);
- }
- /*将缓冲区关联到页面*/
- attach_page_buffers(page, head);
- spin_unlock(&page->mapping->private_lock);
- }
- static inline void attach_page_buffers(struct page *page,
- struct buffer_head *head)
- {
- page_cache_get(page);/*递增引用计数*/
- /*设置PG_private标志,通知内核其他部分,page实例的private成员正在使用中*/
- SetPagePrivate(page);
- /*将页的private成员设置为一个指向环形链表中第一个缓冲头的指针*/
- set_page_private(page, (unsigned long)head);
- }
交互
如果对内核的其他部分无益,那么在页和缓冲区之间建立关联就没起作用。一些与块设备之间的传输操作,传输单位的长度依赖于底层设备的块长度,而内核的许多部分更喜欢按页的粒度来执行I/O操作,因为这使得其他事情更容易处理,特别是内存管理方面。在这种场景下,缓冲头区充当了双方的中介。
从缓冲区中读取整页
首先考察内核在从块设备读取整页时采用的方法,以block_read_full_page为例。我们讨论缓冲区实现所关注的部分。
- /*
- * Generic “read page” function for block devices that have the normal
- * get_block functionality. This is most of the block device filesystems.
- * Reads the page asynchronously — the unlock_buffer() and
- * set/clear_buffer_uptodate() functions propagate buffer state into the
- * page struct once IO has completed.
- */
- int block_read_full_page(struct page *page, get_block_t *get_block)
- {
- struct inode *inode = page->mapping->host;
- sector_t iblock, lblock;
- struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
- unsigned int blocksize;
- int nr, i;
- int fully_mapped = 1;
- BUG_ON(!PageLocked(page));
- blocksize = 1 << inode->i_blkbits;
- /*检查页是否有相关联的缓冲区,如果没有,则创建他*/
- if (!page_has_buffers(page))
- create_empty_buffers(page, blocksize, 0);
- /*获得这些缓冲区,无论是新建的还是已经存在的
- 只是将page的private成员转换为buffer_head指针,因为按照
- 惯例,private指向与page关联的第一个缓冲头*/
- head = page_buffers(page);
- iblock = (sector_t)page->index << (PAGE_CACHE_SHIFT – inode->i_blkbits);
- lblock = (i_size_read(inode)+blocksize-1) >> inode->i_blkbits;
- bh = head;
- nr = 0;
- i = 0;
- /*内核遍历与页面关联的所有缓冲区*/
- do {
- /*如果缓冲区内容是最新的,内核继续处理下一个
- 缓冲区。在这种情况下,页面缓冲区中的数据与块
- 设备匹配,无需额外的读操作*/
- if (buffer_uptodate(bh))
- continue;
- /*如果没有映射*/
- if (!buffer_mapped(bh)) {
- int err = 0;
- fully_mapped = 0;
- if (iblock < lblock) {
- WARN_ON(bh->b_size != blocksize);
- /*确定块在块设备上的位置*/
- err = get_block(inode, iblock, bh, 0);
- if (err)
- SetPageError(page);
- }
- if (!buffer_mapped(bh)) {
- zero_user(page, i * blocksize, blocksize);
- if (!err)
- set_buffer_uptodate(bh);
- continue;
- }
- /*
- * get_block() might have updated the buffer
- * synchronously
- */
- if (buffer_uptodate(bh))
- continue;
- }
- /*如果缓冲区已经建立了与块的映射,但是其内容不是最新
- 的则将缓冲区放置到一个临时的数组中*/
- arr[nr++] = bh;
- } while (i++, iblock++, (bh = bh->b_this_page) != head);
- if (fully_mapped)
- SetPageMappedToDisk(page);
- if (!nr) {
- /*
- * All buffers are uptodate – we can set the page uptodate
- * as well. But not if get_block() returned an error.
- */
- if (!PageError(page))
- SetPageUptodate(page);
- unlock_page(page);
- return 0;
- }
- /* Stage two: lock the buffers */
- for (i = 0; i < nr; i++) {
- bh = arr[i];
- lock_buffer(bh);
- /*将b_end_io设置为end_buffer_async_read,该函数将在数据传输结构时
- 调用*/
- mark_buffer_async_read(bh);
- }
- /*
- * Stage 3: start the IO. Check for uptodateness
- * inside the buffer lock in case another process reading
- * the underlying blockdev brought it uptodate (the sct fix).
- */
- for (i = 0; i < nr; i++) {
- bh = arr[i];
- if (buffer_uptodate(bh))
- end_buffer_async_read(bh, 1);
- else
- /*将所有需要读取的缓冲区转交给块层
- 也就是BIO层,在其中开始读操作*/
- submit_bh(READ, bh);
- }
- return 0;
- }
将整页写入到缓冲区
除了读操作之外,页面的写操作也可以划分为更小的单位。只有页中实际修改的内容需要回写,而不用回写整页的内容。遗憾的是,从缓冲区的角度来看,写操作的实现比上述的读操作复杂的多。
__block_wirte_full_page函数中回写脏页面设计的缓冲区相关操作。
- /*
- * NOTE! All mapped/uptodate combinations are valid:
- *
- * Mapped Uptodate Meaning
- *
- * No No “unknown” – must do get_block()
- * No Yes “hole” – zero-filled
- * Yes No “allocated” – allocated on disk, not read in
- * Yes Yes “valid” – allocated and up-to-date in memory.
- *
- * “Dirty” is valid only with the last case (mapped+uptodate).
- */
- /*
- * While block_write_full_page is writing back the dirty buffers under
- * the page lock, whoever dirtied the buffers may decide to clean them
- * again at any time. We handle that by only looking at the buffer
- * state inside lock_buffer().
- *
- * If block_write_full_page() is called for regular writeback
- * (wbc->sync_mode == WB_SYNC_NONE) then it will redirty a page which has a
- * locked buffer. This only can happen if someone has written the buffer
- * directly, with submit_bh(). At the address_space level PageWriteback
- * prevents this contention from occurring.
- *
- * If block_write_full_page() is called with wbc->sync_mode ==
- * WB_SYNC_ALL, the writes are posted using WRITE_SYNC_PLUG; this
- * causes the writes to be flagged as synchronous writes, but the
- * block device queue will NOT be unplugged, since usually many pages
- * will be pushed to the out before the higher-level caller actually
- * waits for the writes to be completed. The various wait functions,
- * such as wait_on_writeback_range() will ultimately call sync_page()
- * which will ultimately call blk_run_backing_dev(), which will end up
- * unplugging the device queue.
- */
- static int __block_write_full_page(struct inode *inode, struct page *page,
- get_block_t *get_block, struct writeback_control *wbc,
- bh_end_io_t *handler)
- {
- int err;
- sector_t block;
- sector_t last_block;
- struct buffer_head *bh, *head;
- const unsigned blocksize = 1 << inode->i_blkbits;
- int nr_underway = 0;
- int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
- WRITE_SYNC_PLUG : WRITE);
- BUG_ON(!PageLocked(page));
- last_block = (i_size_read(inode) – 1) >> inode->i_blkbits;
- /*页面是否有关联缓冲区,如果没有创建他*/
- if (!page_has_buffers(page)) {
- create_empty_buffers(page, blocksize,
- (1 << BH_Dirty)|(1 << BH_Uptodate));
- }
- /*
- * Be very careful. We have no exclusion from __set_page_dirty_buffers
- * here, and the (potentially unmapped) buffers may become dirty at
- * any time. If a buffer becomes dirty here after we’ve inspected it
- * then we just miss that fact, and the page stays dirty.
- *
- * Buffers outside i_size may be dirtied by __set_page_dirty_buffers;
- * handle that here by just cleaning them.
- */
- block = (sector_t)page->index << (PAGE_CACHE_SHIFT – inode->i_blkbits);
- head = page_buffers(page);
- bh = head;
- /*
- * Get all the dirty buffers mapped to disk addresses and
- * handle any aliases from the underlying blockdev’s mapping.
- */
- /*对所有未映射的脏缓冲区,在缓冲区和块设备
- 之间建立映射*/
- do {
- if (block > last_block) {
- /*
- * mapped buffers outside i_size will occur, because
- * this page can be outside i_size when there is a
- * truncate in progress.
- */
- /*
- * The buffer was zeroed by block_write_full_page()
- */
- clear_buffer_dirty(bh);
- set_buffer_uptodate(bh);
- } else if ((!buffer_mapped(bh) || buffer_delay(bh)) &&
- buffer_dirty(bh)) {
- WARN_ON(bh->b_size != blocksize);
- /*查找块设备上与缓冲区项匹配的块*/
- err = get_block(inode, block, bh, 1);
- if (err)
- goto recover;
- clear_buffer_delay(bh);
- if (buffer_new(bh)) {
- /* blockdev mappings never come here */
- clear_buffer_new(bh);
- unmap_underlying_metadata(bh->b_bdev,
- bh->b_blocknr);
- }
- }
- bh = bh->b_this_page;
- block++;
- } while (bh != head);
- /*第二遍遍历,将滤出所有的脏缓冲区*/
- do {
- if (!buffer_mapped(bh))
- continue;
- /*
- * If it’s a fully non-blocking write attempt and we cannot
- * lock the buffer then redirty the page. Note that this can
- * potentially cause a busy-wait loop from writeback threads
- * and kswapd activity, but those code paths have their own
- * higher-level throttling.
- */
- if (wbc->sync_mode != WB_SYNC_NONE || !wbc->nonblocking) {
- lock_buffer(bh);
- } else if (!trylock_buffer(bh)) {
- redirty_page_for_writepage(wbc, page);
- continue;
- }
- /*如果设置了脏页标志,则会在调用该函数时清除
- 因为缓冲区的内容将立即回写*/
- if (test_clear_buffer_dirty(bh)) {
- /*设置BH_Async_Write状态位,并将end_buffer_async_write
- 指定为BIO完成处理程序即b_end_io*/
- mark_buffer_async_write_endio(bh, handler);
- } else {
- unlock_buffer(bh);
- }
- } while ((bh = bh->b_this_page) != head);
- /*
- * The page and its buffers are protected by PageWriteback(), so we can
- * drop the bh refcounts early.
- */
- BUG_ON(PageWriteback(page));
- set_page_writeback(page);
- /*最后一次遍历*/
- do {
- struct buffer_head *next = bh->b_this_page;
- if (buffer_async_write(bh)) {
- /*将前一次遍历中标记为BH_Async_Write的所有缓冲区
- 转交给块层执行实际的写操作,该函数向块层提交
- 了对应的请求*/
- submit_bh(write_op, bh);
- nr_underway++;
- }
- bh = next;
- } while (bh != head);
- unlock_page(page);
- err = 0;
- done:
- if (nr_underway == 0) {
- /*
- * The page was marked dirty, but the buffers were
- * clean. Someone wrote them back by hand with
- * ll_rw_block/submit_bh. A rare case.
- */
- end_page_writeback(page);
- /*
- * The page and buffer_heads can be released at any time from
- * here on.
- */
- }
- return err;
- recover:
- /*
- * ENOSPC, or some other error. We may already have added some
- * blocks to the file, so we need to write these out to avoid
- * exposing stale data.
- * The page is currently locked and not marked for writeback
- */
- bh = head;
- /* Recovery: lock and submit the mapped buffers */
- do {
- if (buffer_mapped(bh) && buffer_dirty(bh) &&
- !buffer_delay(bh)) {
- lock_buffer(bh);
- mark_buffer_async_write_endio(bh, handler);
- } else {
- /*
- * The buffer may have been set dirty during
- * attachment to a dirty page.
- */
- clear_buffer_dirty(bh);
- }
- } while ((bh = bh->b_this_page) != head);
- SetPageError(page);
- BUG_ON(PageWriteback(page));
- mapping_set_error(page->mapping, err);
- set_page_writeback(page);
- do {
- struct buffer_head *next = bh->b_this_page;
- if (buffer_async_write(bh)) {
- clear_buffer_dirty(bh);
- submit_bh(write_op, bh);
- nr_underway++;
- }
- bh = next;
- } while (bh != head);
- unlock_page(page);
- goto done;
- }