loop device

测试:

sudo mount -o loop test.iso /mnt

dd if=/dev/null of=x.ext4 bs=1000M seek=100
mkfs.ext4 -F x.ext4
mkdir -p dir
sudo mount -t ext4 -o loop x.ext4 dir
sudo chown martins3 dir

sysfs 中的东西:

🧀  pwd
/sys/block/loop0/loop

block/loop0/loop🔒 🐶
 grep . *
autoclear:1
backing_file:/home/martins3/x.ext4
dio:0
offset:0
partscan:0
sizelimit:0

0xffffffff83003bdc in loop_init () at drivers/block/loop.c:2261

• loop_add

static const struct blk_mq_ops loop_mq_ops = {
	.queue_rq       = loop_queue_rq,
	.complete	= lo_complete_rq,
};

这个在 mount 过程中，总是出现的:

• entry_SYSCALL_64
  • do_syscall_64
    • do_syscall_x64
      • __x64_sys_mount
        • __se_sys_mount
          • __do_sys_mount
            • do_mount
              • path_mount
                • do_new_mount
                  • vfs_get_tree
                    • get_tree_bdev
                      • ext4_fill_super
                        • __ext4_fill_super
                          • ext4_load_super
                            • ext4_sb_bread_unmovable
                              • __ext4_sb_bread_gfp
                                • ext4_read_bh
                                  • __ext4_read_bh
                                    • submit_bio_noacct_nocheck
                                      • submit_bio_noacct_nocheck
                                        • __submit_bio_noacct_mq
                                          • __submit_bio
                                            • blk_mq_submit_bio
                                              • blk_mq_try_issue_directly
                                                • __blk_mq_try_issue_directly
                                                  • __blk_mq_issue_directly
                                                    • loop_queue_rq

运行的路线基本

@[
        loop_queue_rq+0
        blk_mq_request_issue_directly+92
        blk_mq_issue_direct+140
        blk_mq_dispatch_queue_requests+296
        blk_mq_flush_plug_list+152
        __blk_flush_plug+248
        blk_finish_plug+64
        __iomap_dio_rw+552
        iomap_dio_rw+24
        ext4_file_write_iter+804
        vfs_write+548
        ksys_write+120
        __arm64_sys_write+36
        invoke_syscall.constprop.0+88
        do_el0_svc+72
        el0_svc+92
        el0t_64_sync_handler+268
        el0t_64_sync+408
]: 45241

• ret_from_fork
  • kthread
    • worker_thread
      • process_one_work
        • loop_process_work
          • loop_handle_cmd
            • do_req_filebacked
              • lo_write_simple
                • lo_write_bvec

loop 可以让 md 是基于 file 的，真神奇啊

• https://unix.stackexchange.com/questions/302766/persistent-use-of-loop-block-device-in-mdadm
• https://stackoverflow.com/questions/4519761/programming-a-loopback-device-consisting-of-several-files-in-linux

简单来说，loop 就是让 mount 一个文件作为盘的，就像是 swap file 一样

• loop_queue_rq
  • loop_queue_work
    • loop_process_work
      • 最后一路达到 lo_write_bvec

原来容量是可以设置的

https://serverfault.com/questions/690192/why-is-my-loop-device-size-0

losetup –set-capacity /dev/loop0 blockdev –getsize64 /dev/loop0

可以主动将设备的容量设置为 0 吗?

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