New file [[hurd/io_path]], based on an email by Samuel Thibault, bug-hurd, 2008-03-27.

2 files changed, 38 insertions, 0 deletions

diff --git a/hurd.mdwn b/hurd.mdwn
index 1edf4d55..ff1f2916 100644
--- a/hurd.mdwn
+++ b/hurd.mdwn
@@ -64,5 +64,6 @@
   * [[libstore]]
   * [[libchannel]]
   * [[libhello_example]] -- Hurd library example
+* [[IO_Path]]
 * [[Debugging]]
 * [Hurd Sourcecode Reference](http://www.htu.tugraz.at/~past/hurd/global/): Searchable and browsable index of the code.
diff --git a/hurd/io_path.mdwn b/hurd/io_path.mdwn
new file mode 100644
index 00000000..96e6aa58
--- /dev/null
+++ b/hurd/io_path.mdwn
@@ -0,0 +1,37 @@
+[[meta copyright="Copyright © 2008 Free Software Foundation, Inc."]]
+
+[[meta license="""[[toggle id="license" text="GFDL 1.2+"]][[toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
+is included in the section entitled
+[[GNU_Free_Documentation_License|/fdl]]."]]"""]]
+
+# 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`.