Ramdisk源代码分析

#include #include #include #include #include #include #include #include #include #include #include        /* for invalidate_bdev() */#include #include #include

#include

/* Various static variables go here. Most are used only in the RAM disk code.*/

static struct gendisk *rd_disks[CONFIG_BLK_DEV_RAM_COUNT];    //the default value is 16static struct block_device *rd_bdev[CONFIG_BLK_DEV_RAM_COUNT];/* Protected device data */static struct request_queue *rd_queue[CONFIG_BLK_DEV_RAM_COUNT];

/** Parameters for the boot-loading of the RAM disk. These are set by* init/main.c (from arguments to the kernel command line) or from the* architecture-specific setup routine (from the stored boot sector* information).*/int rd_size = CONFIG_BLK_DEV_RAM_SIZE; // 4M     /* Size of the RAM disks *//** It would be very desirable to have a soft-blocksize (that in the case* of the ramdisk driver is also the hardblocksize ;) of PAGE_SIZE because* doing that we'll achieve a far better MM footprint. Using a rd_blocksize of* BLOCK_SIZE in the worst case we'll make PAGE_SIZE/BLOCK_SIZE buffer-pages* unfreeable. With a rd_blocksize of PAGE_SIZE instead we are sure that only* 1 page will be protected. Depending on the size of the ramdisk you* may want to change the ramdisk blocksize to achieve a better or worse MM* behaviour. The default is still BLOCK_SIZE (needed by rd_load_image that* supposes the filesystem in the image uses a BLOCK_SIZE blocksize).*/static int rd_blocksize = CONFIG_BLK_DEV_RAM_BLOCKSIZE; //the block size

/** Copyright (C) 2000 Linus Torvalds.*               2000 Transmeta Corp.* aops copied from ramfs.*/

/** If a ramdisk page has buffers, some may be uptodate and some may be not.* To bring the page uptodate we zero out the non-uptodate buffers. The* page must be locked.*/static void make_page_uptodate(struct page *page){   if (page_has_buffers(page)) {       struct buffer_head *bh = page_buffers(page);       struct buffer_head *head = bh;

       do {           if (!buffer_uptodate(bh)) {               memset(bh->b_data, 0, bh->b_size);               /*               * akpm: I'm totally undecided about this. The               * buffer has just been magically brought "up to               * date", but nobody should want to be reading               * it anyway, because it hasn't been used for               * anything yet. It is still in a "not read               * from disk yet" state.               *               * But non-uptodate buffers against an uptodate               * page are against the rules. So do it anyway.               */               set_buffer_uptodate(bh);           }       } while ((bh = bh->b_this_page) != head);   } else {       memset(page_address(page), 0, PAGE_CACHE_SIZE);   }   flush_dcache_page(page);   SetPageUptodate(page);}

static int ramdisk_readpage(struct file *file, struct page *page){   if (!PageUptodate(page))       make_page_uptodate(page);   unlock_page(page);   return 0;}

static int ramdisk_prepare_write(struct file *file, struct page *page,               unsigned offset, unsigned to){   if (!PageUptodate(page))       make_page_uptodate(page);   return 0;}

static int ramdisk_commit_write(struct file *file, struct page *page,               unsigned offset, unsigned to){   set_page_dirty(page);   return 0;}

/** ->writepage to the the blockdev's mapping has to redirty the page so that the* VM doesn't go and steal it. We return AOP_WRITEPAGE_ACTIVATE so that the VM* won't try to (pointlessly) write the page again for a while.** Really, these pages should not be on the LRU at all.*/static int ramdisk_writepage(struct page *page, struct writeback_control *wbc){   if (!PageUptodate(page))       make_page_uptodate(page);   SetPageDirty(page);   if (wbc->for_reclaim)       return AOP_WRITEPAGE_ACTIVATE;   unlock_page(page);   return 0;}

/** This is a little speedup thing: short-circuit attempts to write back the* ramdisk blockdev inode to its non-existent backing store.*/static int ramdisk_writepages(struct address_space *mapping,               struct writeback_control *wbc){   return 0;}

/** ramdisk blockdev pages have their own ->set_page_dirty() because we don't* want them to contribute to dirty memory accounting.*/static int ramdisk_set_page_dirty(struct page *page){   if (!TestSetPageDirty(page))       return 1;   return 0;}

//块设备的address_space_operations结构，注意与文件系统的相应结果的区别。static const struct address_space_operations ramdisk_aops = {   .readpage   = ramdisk_readpage,   .prepare_write   = ramdisk_prepare_write,   .commit_write   = ramdisk_commit_write,   .writepage   = ramdisk_writepage,   .set_page_dirty   = ramdisk_set_page_dirty,   .writepages   = ramdisk_writepages,};

static int rd_blkdev_pagecache_IO(int rw, struct bio_vec *vec, sector_t sector,               struct address_space *mapping){   pgoff_t index = sector >> (PAGE_CACHE_SHIFT - 9);   unsigned int vec_offset = vec->bv_offset;   int offset = (sector bv_len;   int err = 0;

   do {       int count;       struct page *page;       char *src;       char *dst;

       count = PAGE_CACHE_SIZE - offset;       if (count > size)           count = size;       size -= count;

       //从页面缓存中读，如果没有，则出错，而不像对于ide-disk的处理，它出错会导致从磁盘中读入。       page = grab_cache_page(mapping, index);       if (!page) {           err = -ENOMEM;           goto out;       }

       if (!PageUptodate(page))           make_page_uptodate(page);

       index++;

       //建立映射       if (rw == READ) {           src = kmap_atomic(page, KM_USER0) + offset;           dst = kmap_atomic(vec->bv_page, KM_USER1) + vec_offset;       } else {           src = kmap_atomic(vec->bv_page, KM_USER0) + vec_offset;           dst = kmap_atomic(page, KM_USER1) + offset;       }       offset = 0;       vec_offset += count;

       memcpy(dst, src, count); //拷贝

       //临时映射必须在使用完后立即解除映射       kunmap_atomic(src, KM_USER0);       kunmap_atomic(dst, KM_USER1);

       if (rw == READ)           flush_dcache_page(vec->bv_page);       else           set_page_dirty(page);   //使页面不可被释放       unlock_page(page);       put_page(page);   } while (size);

out:   return err;}

/** Basically, my strategy here is to set up a buffer-head which can't be* deleted, and make that my Ramdisk. If the request is outside of the* allocated size, we must get rid of it...** 19-JAN-1998 Richard Gooch Added devfs support**///这是个处理BIO请求的函数，对于IDE-DISK相应的函数是通过IO-scheduler将待处理的BIO请求加入到请求队列中，或将这个BIO合并到合适的请求(request)中，在做下一步的处理。而现在对于ramdisk这个随机访问设备，IO-scheduler则不必要，所以这个函数直接处理的BIO请求。

static int rd_make_request(request_queue_t *q, struct bio *bio){   struct block_device *bdev = bio->bi_bdev;   struct address_space * mapping = bdev->bd_inode->i_mapping;   sector_t sector = bio->bi_sector;   unsigned long len = bio->bi_size >> 9;   int rw = bio_data_dir(bio);   struct bio_vec *bvec;   int ret = 0, i;

   if (sector + len > get_capacity(bdev->bd_disk))       goto fail;

   if (rw==READA) //预备读处理       rw=READ;

   bio_for_each_segment(bvec, bio, i) { //处理BIO中的每一个segment。       ret |= rd_blkdev_pagecache_IO(rw, bvec, sector, mapping);       sector += bvec->bv_len >> 9;   }   if (ret)       goto fail;

   bio_endio(bio, bio->bi_size, 0);   return 0;fail:   bio_io_error(bio, bio->bi_size);   return 0;}

static int rd_ioctl(struct inode *inode, struct file *file,           unsigned int cmd, unsigned long arg){   int error;   struct block_device *bdev = inode->i_bdev;

   if (cmd != BLKFLSBUF)       return -ENOTTY;

   /*   * special: we want to release the ramdisk memory, it's not like with   * the other blockdevices where this ioctl only flushes away the buffer   * cache   */   error = -EBUSY;   mutex_lock(&bdev->bd_mutex);   if (bdev->bd_openers bd_inode->i_mapping, 0);       error = 0;   }   mutex_unlock(&bdev->bd_mutex);   return error;}

/** This is the backing_dev_info for the blockdev inode itself. It doesn't need* writeback and it does not contribute to dirty memory accounting.*/static struct backing_dev_info rd_backing_dev_info = {   .ra_pages   = 0,   /* No readahead */   .capabilities   = BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK | BDI_CAP_MAP_COPY,   .unplug_io_fn   = default_unplug_io_fn,};

/** This is the backing_dev_info for the files which live atop the ramdisk* "device". These files do need writeback and they do contribute to dirty* memory accounting.*/static struct backing_dev_info rd_file_backing_dev_info = {   .ra_pages   = 0,   /* No readahead */   .capabilities   = BDI_CAP_MAP_COPY,   /* Does contribute to dirty memory */   .unplug_io_fn   = default_unplug_io_fn,};

static int rd_open(struct inode *inode, struct file *filp){   unsigned unit = iminor(inode);

   if (rd_bdev[unit] == NULL) {       struct block_device *bdev = inode->i_bdev;       struct address_space *mapping;       unsigned bsize;       gfp_t gfp_mask;

       inode = igrab(bdev->bd_inode);       rd_bdev[unit] = bdev;       bdev->bd_openers++;       bsize = bdev_hardsect_size(bdev);       bdev->bd_block_size = bsize;       inode->i_blkbits = blksize_bits(bsize);       inode->i_size = get_capacity(bdev->bd_disk)a_ops = &ramdisk_aops;       mapping->backing_dev_info = &rd_backing_dev_info;       bdev->bd_inode_backing_dev_info = &rd_file_backing_dev_info;

       /*       * Deep badness. rd_blkdev_pagecache_IO() needs to allocate       * pagecache pages within a request_fn. We cannot recur back       * into the filesytem which is mounted atop the ramdisk, because       * that would deadlock on fs locks. And we really don't want       * to reenter rd_blkdev_pagecache_IO when we're already within       * that function.       *       * So we turn off __GFP_FS and __GFP_IO.       *       * And to give this thing a hope of working, turn on __GFP_HIGH.       * Hopefully, there's enough regular memory allocation going on       * for the page allocator emergency pools to keep the ramdisk       * driver happy.       */       gfp_mask = mapping_gfp_mask(mapping);       gfp_mask &= ~(__GFP_FS|__GFP_IO);       gfp_mask |= __GFP_HIGH;       mapping_set_gfp_mask(mapping, gfp_mask);   }

   return 0;}

static struct block_device_operations rd_bd_op = {   .owner =   THIS_MODULE,   .open =       rd_open,   .ioctl =   rd_ioctl,};

/** Before freeing the module, invalidate all of the protected buffers!*/static void __exit rd_cleanup(void){   int i;

   for (i = 0; i < CONFIG_BLK_DEV_RAM_COUNT; i++) {       struct block_device *bdev = rd_bdev[i];       rd_bdev[i] = NULL;       if (bdev) {           invalidate_bdev(bdev, 1);           blkdev_put(bdev);       }       del_gendisk(rd_disks[i]);       put_disk(rd_disks[i]);       blk_cleanup_queue(rd_queue[i]);   }   unregister_blkdev(RAMDISK_MAJOR, "ramdisk");}

/** This is the registration and initialization section of the RAM disk driver*/static int __init rd_init(void){   int i;   int err = -ENOMEM;

   //检查块大小是否合适，它必须小于一个内存页面的大小，且要大于512字节，还要是2的N次幂。   if (rd_blocksize > PAGE_SIZE || rd_blocksize < 512 ||           (rd_blocksize & (rd_blocksize-1))) {       printk("RAMDISK: wrong blocksize %d, reverting to defaults\n",               rd_blocksize);       rd_blocksize = BLOCK_SIZE; //如果不是则使用默认值   }

   for (i = 0; i < CONFIG_BLK_DEV_RAM_COUNT; i++) {       rd_disks[i] = alloc_disk(1);   //分配硬盘描述符       if (!rd_disks[i])           goto out;

       rd_queue[i] = blk_alloc_queue(GFP_KERNEL); //分配请求队列描述符       if (!rd_queue[i]) {           put_disk(rd_disks[i]);           goto out;       }   }

   //注册进内核   if (register_blkdev(RAMDISK_MAJOR, "ramdisk")) {       err = -EIO;       goto out;   }

   //初始化请求队列，并将磁盘加入到队列中去   for (i = 0; i < CONFIG_BLK_DEV_RAM_COUNT; i++) {       struct gendisk *disk = rd_disks[i];

   //注册请求队列的BIO请求函数   blk_queue_make_request(rd_queue[i], &rd_make_request);       //盘块大小blk_queue_hardsect_size(rd_queue[i], rd_blocksize);

       /* rd_size is given in kB */       //以下两行注册设备的主次号       disk->major = RAMDISK_MAJOR; //对于RAMDISK，主设备号都是这个       disk->first_minor = i;    //次设备号标志是第几个盘       disk->fops = &rd_bd_op;       disk->queue = rd_queue[i]; //每个盘一个请求队列。       disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;       sprintf(disk->disk_name, "ram%d", i);       set_capacity(disk, rd_size * 2);       add_disk(rd_disks[i]); //加入，至此，该盘可以被访问   }

   /* rd_size is given in kB */   printk("RAMDISK driver initialized: "       "%d RAM disks of %dK size %d blocksize\n",       CONFIG_BLK_DEV_RAM_COUNT, rd_size, rd_blocksize);

   return 0;out:   while (i--) {       put_disk(rd_disks[i]);       blk_cleanup_queue(rd_queue[i]);   }   return err;}

module_init(rd_init);module_exit(rd_cleanup);

/* options - nonmodular */#ifndef MODULEstatic int __init ramdisk_size(char *str){   rd_size = simple_strtol(str,NULL,0);   return 1;}static int __init ramdisk_size2(char *str)   /* kludge */{   return ramdisk_size(str);}static int __init ramdisk_blocksize(char *str){   rd_blocksize = simple_strtol(str,NULL,0);   return 1;}__setup("ramdisk=", ramdisk_size); //为向前兼容而存在，等于ramdisk_size__setup("ramdisk_size=", ramdisk_size2);__setup("ramdisk_blocksize=", ramdisk_blocksize);#endif

/* options - modular */module_param(rd_size, int, 0);MODULE_PARM_DESC(rd_size, "Size of each RAM disk in kbytes.");module_param(rd_blocksize, int, 0);MODULE_PARM_DESC(rd_blocksize, "Blocksize of each RAM disk in bytes.");MODULE_ALIAS_BLOCKDEV_MAJOR(RAMDISK_MAJOR);

MODULE_LICENSE("GPL");