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[[!meta title="I/O Path"]]
# read
* [[glibc]]'s `read` is in `glibc/sysdeps/mach/hurd/read.c:__libc_read`.
* That calls `glibc/hurd/fd-read.c:_hurd_fd_read()`.
* That calls `__io_read`, which is an [[RPC]], i.e., that actually results
into the [[translator/ext2fs]] server calling
`hurd/libdiskfs/io-read.c:diskfs_S_io_read`.
* That calls `_diskfs_rdwr_internal`, which calls
`hurd/libpager/pager-memcpy.c:pager_memcpy`, which usually basically just
tell the kernel to virtually project the memory object corresponding to the
file in the caller process's memory. No read is actually done.
* Then, when the process actually reads the data, the kernel gets the user
page fault (`gnumach/i386/i386/trap.c:user_trap`), which calls `vm_fault`,
etc., until actually getting to `gnumach/vm/vm_fault/vm_fault_page` which
eventually calls `memory_object_data_request`, which is an [[RPC]], i.e.,
that actually results into the [[translator/ext2fs]] server calling
`hurd/libpager/data-request.c:_pager_seqnos_memory_object_data_request`.
* That calls `hurd/ext2fs/pager.c:pager_read_page`, which looks for where the
data is on the disk, and eventually calls
`hurd/libstore/rdwr.c:store_read`, which eventually calls `device_read`,
which is an [[RPC]], i.e., that actually gets into the kernel calling
`gnumach/linux/dev/glue/block.c:device_read`.
* ext2fs eventually finishes the data_request() function, the kernel installs
the page into the process that got a fault.
# Documentation
* In [*Linux kernel design patterns - part
3*](http://lwn.net/Articles/336262/) (2009-06-22), Neil Brown gives a
nice overview of the related layering inside the Linux kernel,
including the VFS layer, page cache and directory entry cache
(dcache).
[[!tag open_issue_documentation]] <!-- Merge kepler.SCHWINGE/master-io_path.
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