diff options
| -rw-r--r-- | hurd/io_path.mdwn | 83 |
1 files changed, 68 insertions, 15 deletions
diff --git a/hurd/io_path.mdwn b/hurd/io_path.mdwn index 0d83a4ba..492edffe 100644 --- a/hurd/io_path.mdwn +++ b/hurd/io_path.mdwn @@ -1,4 +1,5 @@ -[[!meta copyright="Copyright © 2008, 2010 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2008, 2010, 2011 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 @@ -10,21 +11,77 @@ License|/fdl]]."]]"""]] [[!meta title="I/O Path"]] +[[!tag open_issue_documentation]] <!-- Someone still needs to make a pass over +this text. --> -# read +[[!toc]] - * [[glibc]]'s `read` is in `glibc/sysdeps/mach/hurd/read.c:__libc_read`. - * That calls `glibc/hurd/fd-read.c:_hurd_fd_read()`. +# `read`, [[libtrivfs]] - * 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`. +[[glibc]]'s `read` is in `glibc/sysdeps/mach/hurd/read.c:__libc_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. +A buffer (and its size) to store the to-be-read data in is supplied by the +caller of `read`. + +> `__libc_read` calls `glibc/hurd/fd-read.c:_hurd_fd_read`. + +>> `_hurd_fd_read` calls `__io_read`, which is an [[RPC]]: +>> `hurd/hurd/io.defs:io_read`. + +>>> Enter user-side RPC stub `glibc.obj/hurd/RPC_io_read.c:__io_read`. Process +>>> stuff, switch to kernel, etc. + +(For example) [[translator/hello]] server, [[libtrivfs]]-based. Enter +server-side RPC stub `hurd.obj/libtrivfs/ioServer.c:_Xio_read`. Process stuff, +call `hurd/trans/hello.c:trivfs_S_io_read`. + +A 2048 byte buffer is provided. + +> `trivfs_S_io_read`. Depending on the internal state, either a new memory +> region is set-up (and returned as out-of-line data), or the desired amount of +> data is returned in-line. + +Back in `_Xio_read`. + +If the 2048 byte buffer is not decided to be used (out-of-line case or bigger +than 2048 bytes case; server decides to instead provide a new memory region), +the [[`dealloc`|microkernel/mach/mig/dealloc]] flag is being set, which causes +Mach to unmap that memory region from the server's address space, i.e., doing a +memory *move* from the server to the client. + +Leave server-side RPC stub `_Xio_read`. + +>>> Return from kernel, continue client-side RPC stub `io_read`. Have to copy +>>> data. Three cases: out-of-line data (pass pointer to memory area); +>>> returned more data than fits into the originally supplied buffer (allocate +>>> new buffer, copy all data into it, pass pointer of new buffer); otherwise +>>> copy as much data as is available into the originally supplied buffer. +>>> I.e., in all cases *all* data which was provided by the server is made +>>> available to the caller. + +>> Back in `_hurd_fd_read`. If a new buffer has been allocated previously, or +>> the out-of-line mechanism has been used, the returned data now has to be +>> copied into the originally supplied buffer. If the server returned more +>> data than requested, this is a [[protocol_violation|EGRATUITOUS]]. + +> Back in `__libc_read`. + + +# `read`, [[hurd/translator/ext2fs]]/[[hurd/libdiskfs]] + +(For example) [[translator/ext2fs]] server, enter server-side RPC stub +`hurd.obj/libdiskfs/ioServer.c:_Xio_read`. Process stuff, call +`hurd/libdiskfs/io-read.c:diskfs_S_io_read`. + +A 2048 byte buffer is provided. + +> `diskfs_S_io_read` calls `_diskfs_rdwr_internal`. + +>> That calls `hurd/libpager/pager-memcpy.c:pager_memcpy`, which usually +>> basically just tells 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`, @@ -50,7 +107,3 @@ License|/fdl]]."]]"""]] 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. ---> |