IO路径解析
首先，得看发出了哪些系统调用， strace出场
我们关心的两个系统调用是open和write
open("/mnt/sdb/zengxi/iopath/hello_file" O_WRONLY|O_CREAT 03777701360411750) = 3 //返回值是fd的值
write(3 "hello world!\n" 13) = 13 //返回值是write系统调用写buf的长度然后，我们一起看看在内核里面做了什么事情
a.out(3594) close: args: 255
a.out(3594) vfs_read parms: file=0xffff880005edc2c0 buf=0xffff880052c4c800 count=0x188 pos=0xffff88005e6e9c68
a.out(3594) open: args: "/etc/ld.so.cache" O_RDONLY
a.out(3594) vfs_fstat parms: fd=0x3 stat=0xffff88005e6e9ef8
a.out(3594) vfs_getattr parms: mnt=0xffff8801b888e180 dentry=0xffff8800137e0a20 stat=0xffff88005e6e9ef8
a.out(3594) close: args: 3
a.out(3594) open: args: "/lib64/libc.so.6" O_RDONLY
a.out(3594) read: args: 3 0x7ffcd5fa2b78 832
a.out(3594) vfs_read parms: file=0xffff8801b1c39bc0 buf=0x7ffcd5fa2b78 count=0x340 pos=0xffff88005e6e9f50
a.out(3594) vfs_fstat parms: fd=0x3 stat=0xffff88005e6e9ef8
a.out(3594) vfs_getattr parms: mnt=0xffff8801b888e180 dentry=0xffff8801b84ee3a0 stat=0xffff88005e6e9ef8
a.out(3594) close: args: 3
a.out(3594) open: args: "/mnt/sdb/zengxi/iopath/hello_file" O_WRONLY|O_CREAT 01777777777772576431770
a.out(3594) vfs_fstat parms: fd=0x1 stat=0xffff88005e6e9ef8
a.out(3594) vfs_getattr parms: mnt=0xffff8801b888e280 dentry=0xffff8801a8948250 stat=0xffff88005e6e9ef8
a.out(3594) write: args: 1 "the address of msg: 0x7ffcd5fa32f0 sizeof ms"... 51
a.out(3594) vfs_write parms: file=0xffff8801b5733580 buf=0x7f9ad453f000 count=0x33 pos=0xffff88005e6e9f50
a.out(3594) vfs_fstat parms: fd=0x0 stat=0xffff88005e6e9ef8
a.out(3594) vfs_getattr parms: mnt=0xffff8801b888e280 dentry=0xffff8801a8948250 stat=0xffff88005e6e9ef8
a.out(3594) read: args: 0 0x7f9ad453e000 1024
a.out(3594) vfs_read parms: file=0xffff8801b5733580 buf=0x7f9ad453e000 count=0x400 pos=0xffff88005e6e9f50hello world已经被写入到Page Cache当中
============================================================================================================================================
vfs_write
generic_file_aio_write
generic_file_direct_write //direct写
generic_file_buffered_write //buffer写
generic_perform_write
write_begin (address_space_operations->write_begin) //依据具体的文件系统定义的write_begin和write_end
iov_iter_copy_from_user_atomic //从用户空间将buf拷贝到内核空间，相当于进入page cache
write_end (address_space_operations->write_end)
ext2/ext3/ext4
File.c (fs\ext2): .aio_write = generic_file_aio_write
File.c (fs\ext3): .aio_write = generic_file_aio_write
File.c (fs\ext4): .aio_write = ext4_file_write
ocfs2
File.c (fs\ocfs2): .aio_write = ocfs2_file_aio_write
============================================================================================================================================fluser进程刷写脏页
============================================================================================================================================
bdi_forker_thread
task = kthread_create(bdi_writeback_thread &bdi->wb "flush-%s" dev_name(bdi->dev)); //backing-dev.c，创建针对块设备的flush进程
bdi_writeback_thread
wb_do_writeback
wb_writeback
__writeback_inodes_sb //if （work->sb）
writeback_inodes_wb
wb_inode //获取要回写的inode
writeback_sb_inodes //回写属于这个superblock的所有inode
writeback_single_inode //分解写单个inode
do_writepages
mapping->a_ops->writepages //如果文件系统自定义了writepages的方法，调用之
generic_writepages //否则，使用通用的writepages方法
__writepage
mapping->a_ops->writepage //调用文件系统自定义的写单个页面的方法，必需实现
============================================================================================================================================回到具体文件系统中，将page内容封装成块层需要的buffer_head和bio，向下提交
ext2/ext3/ext4
Inode.c (fs\ext2): .writepage = ext2_writepage
Inode.c (fs\ext3): .writepage = ext3_writeback_writepage
Inode.c (fs\ext4): .writepage = ext4_writepage
ocfs2
Aops.c (fs\ocfs2): .writepage = ocfs2_writepage
============================================================================================================================================
ext3_writeback_writepage
block_write_full_page //提交整个page
block_write_full_page_endio //并注册io完成的回调函数是end_buffer_async_write
__block_write_full_page
mark_buffer_async_write_endio //将page中所有的buffer_head的设置io返回回调函数（因为一个page当中可能包含多个io，每个io都需要设置end_io函数）
submit_bh //遍历page中的所有buffer_head，调用submit_bh提交
_submit_bh
submit_bio //提交到块层
============================================================================================================================================经submit_bio后，io进入到块层 进入块层之后，文件的概念就彻底消失了，只有块，准确的说是扇区。 为什么要设置块层，主要的目的是 – IO调度 (noop deadline cfq)所在的位置
============================================================================================================================================
submit_bio
generic_make_request //将io请求封装成request
__generic_make_request
q = bdev_get_queue(bio->bi_bdev); //获取到这个设备的请求队列(request_queue)
ret = q->make_request_fn(q bio); //调用make_request_fn出队列
============================================================================================================================================