Linux 内核写文件过程
为了简化块设备驱动分析，我们以Ramdisk块设备为基础，分析Linux内核写数据过程。
本文实例将/dev/ram1格式化为ext4文件系统，然后读写文件。下面是写文件内核栈信息，
从该栈信息中，可以看出整个I/O写操作，所经历的流程。
Pid:4318,comm:cpTainted:G‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐HT2.6.32279.debug#19
CallTrace:
[<ffffffff8135ce59>]?brd_make_request+0x439/0x510
[<ffffffff814fd503>]?printk+0x41/0x46
[<ffffffff81256f64>]?generic_make_request+0x2c4/0x5b0
[<ffffffff8125730c>]?submit_bio+0xbc/0x160
[<ffffffff811acd46>]?submit_bh+0xf6/0x150
[<ffffffffa0109773>]?ext4_mb_init_cache+0x883/0x9f0[ext4]
[<ffffffff8112b560>]?__lru_cache_add+0x40/0x90
[<ffffffffa01099fe>]?ext4_mb_init_group+0x11e/0x210 [ext4]
[<ffffffffa0109bbd>]?ext4_mb_good_group+0xcd/0x110[e
xt4]
[<ffffffffa010b35b>]?ext4_mb_regular_allocator+0x19b/0 x410[ext4]
[<ffffffff814feffe>]?mutex_lock+0x1e/0x50
[<ffffffffa010d22d>]?ext4_mb_new_blocks+0x38d/0x560[ext4]
[<ffffffffa0100ace>]?ext4_ext_find_extent+0x2be/0x320[ext4]
[<ffffffffa0103b83>]?ext4_ext_get_blocks+0x1113/0x1a10[ext4]
[<ffffffff810097cc>]?__switch_to+0x1ac/0x320
[<ffffffff81136959>]?zone_statistics+0x99/0xc0
[<ffffffff811259f1>]?get_page_from_freelist+0x3d1/0x820
[<ffffffffa00dfd39>]?ext4_get_blocks+0xf9/0x2a0[ext4]
[<ffffffffa00e07ad>]?ext4_get_block+0xbd/0x120[ext4]
[<ffffffff811afe1b>]?__block_prepare_write+0x1db/0x570
[<ffffffffa00e06f0>]?ext4_get_block+0x0/0x120[ext4]
[<f
fffffff811b048c>]?block_write_begin_newtrunc+0x5c/0xd0
[<ffffffff811b0893>]?block_write_begin+0x43/0x90
[<ffffffffa00e06f0>]?ext4_get_block+0x0/0x120[ext4]
[<ffffffffa00e4896>]?ext4_write_begin+0x226/0x2d0[ext4]
[<ffffffffa00e06f0>]?ext4_get_block+0x0/0x120[ext4]
[<ffffffffa00e4ac8>]?ext4_da_write_begin+0x188/0x200[ext4]
[<ffffffffa00ab63f>]?jbd2_journal_dirty_metadata+0xff/0x150[jbd2]
[<ffffffff814ff6f6>]?down_read+0x16/0x30
[<ffffffff81114ab3>]?generic_file_buffered_write+0x123/0x2e0
[<ffffffff810724c7>]?current_fs_time+0x27/0x30
[<ffffffff81116450>]?__generic_file_aio_write+0x250/0x480
[<ffffffff8113f1c7>]?handle_pte_fault+0xf7/0xb50
[<ffffffff811166ef>]?generic_file_aio_write+0x6f/0xe0
[<f
fffffffa00d9131>]?ext4_file_write+0x61/0x1e0[ext4]
[<ffffffff8117ad6a>]?do_sync_write+0xfa/0x140
[<ffffffff810920d0>]?autoremove_wake_function+0x0/0x40
[<ffffffff810edfc2>]?ring_buffer_lock_reserve+0xa2/0x160
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