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I know that the Block I/O layer in Linux I/O stack manages and schedules I/O requests. but I do not know where this code is. indeed, I want to see the section of code in kernel which corresponds to this layer.

2

The scheduling of IO starts at the call of the blk_finish_plug function, that itself will call the elevator_add_req_fn method of the IO scheduler.

This is an example of a writeback tread calling this function to add new IOs:

#0  deadline_add_request at block/deadline-iosched.c:102
#1  __elv_add_request at block/elevator.c:717
#2  blk_flush_plug_list at block/blk-core.c:2746
#3  blk_finish_plug at block/blk-core.c:2762
#4  generic_writepages at mm/page-writeback.c:1052
#5  do_writepages at mm/page-writeback.c:1065
#6  writeback_single_inode at fs/fs-writeback.c:388
#7  writeback_sb_inodes at fs/fs-writeback.c:549
#8  writeback_inodes_wb at fs/fs-writeback.c:592
#9  wb_writeback at fs/fs-writeback.c:724
#10 wb_check_background at fs/fs-writeback.c:804
#11 wb_do_writeback at fs/fs-writeback.c:879
#12 bdi_writeback_thread at fs/fs-writeback.c:913
#13 kthread at kernel/kthread.c:96

You can have a look at the files mentioned in the stack trace, especially the blk_finish_plug method of the blk-core.c file. You can find more details on how plugging works at https://lwn.net/Articles/438256/

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