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+[[!meta copyright="Copyright © 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
+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]]."]]"""]]
+
+[[!toc]]
+#Notes on tmpfs
+
+## mach-defpager
+
+[[defpager|http://www.bddebian.com:8888/~hurd-web/user/Maksym_Planeta/#defpager81111]]
+
+[[http://www.mail-archive.com/bug-hurd@gnu.org/msg18859.html]]
+
+## Steps
+
+1. Find out what causes crashes in tmpfs with defpager
+
+[[http://www.gnu.org/s/hurd/hurd/translator/tmpfs/notes_various.html]]
+
+TODO: Consider deleting of parameter "port" in function mach-defpager/default_pager.c:pager_port_list_insert
+since this parameter is unused
+
+2. Write own pager
+
+    6.11.11 Reading/writing for files that fit in vm_page_size works
+
+    7.11.11 Works for any size.
+
+3. Make links work
+
+    Symlinks behavior: [[links|http://www.bddebian.com:8888/~hurd-web/user/Maksym_Planeta/#links81111]]
+
+    8.11.11 Symlinks work.
+
+4. Control of used space by tmpfs.
+
+    TODO: Make tmpfs use not more space than it was allowed.
+
+5. Thread safety.
+
+    TODO: During execution tmpfs hangs in random places. The most possible is variant is deadlocks,
+    because nothing was undertaken for thread safety.
+
+6. After sometime of inactivity tmpfs exits.
+
+    TODO: Find out why and correct this.
+
+#Debugging
+
+To debug tmpfs, using libraries from "$PWD"/lib and trace rpc:
+
+    settrans -ca foo /usr/bin/env LD_LIBRARY_PATH="$PWD"/lib utils/rpctrace -I /usr/share/msgids/ tmpfs/tmpfs 1M
+    LD_LIBRARY_PATH="$PWD"/lib gdb tmpfs/tmpfs `pidof tmpfs` 
+
+For debugging ext2fs:
+
+    settrans --create --active ramdisk0 /hurd/storeio -T copy zero:32M  && \
+    /sbin/mkfs.ext2 -F -b 4096 ramdisk0 && \
+    settrans --active --orphan ramdisk0 /usr/bin/env LD_LIBRARY_PATH="$PWD"/lib utils/rpctrace -I /usr/share/msgids/ \
+    ext2fs/ext2fs.static ramdisk0
+
+#Questions
+
+1. What are sequence numbers? What are they used for?
+2. Is there any way to debug mach-defpager? When I set breakpoint to any function in it, pager never breaks.
+
+#Conversations
+
+## 8.11.11
+
+### links<a id="links81111"/>
+    (10:29:11) braunr: mcsim: ln -s foo/bar foo/baz means the link name is baz in the foo directory,
+    and its target (relative to its directory) is foo/bar (which would mean /tmp/foo/foo/bar in canonical form)
+    (10:29:42) braunr: youpi: tschwinge: what did ludovic achieve ?
+    (10:30:06) tschwinge: mcsim: As Richard says, symlink targets are always relative to the directory they're contained in.
+    (10:30:51) tschwinge: braunr: This Hydra/Nix (I wtill mix it all up) thing is kind of a package managing system.
+    (10:31:17) tschwinge: He has written scripts for bootstrapping a Hurd toolchain.
+    (10:31:26) braunr: oh ok
+    (10:31:27) mcsim: so, if I want to create link in cd, first I need to cd there?
+    (10:31:28) tschwinge: And then uses that to build a whole bootable image.
+    (10:31:36) mcsim: in foo*
+    (10:31:36) braunr: mcsim: just provide the right paths
+    (10:32:11) braunr: $ touch foo/bar
+    (10:32:14) braunr: $ ln -s bar foo/baz
+    (10:32:32) braunr: bar
+    (10:32:35) braunr: baz -> bar
+
+### defpager<a id="defpager81111"/>
+
+    earlier:
+    <tschwinge>: 1. On every system there is a ``default pager'' (mach-defpager).  That one is responsible 
+    for all ``anonymous memory''.  For example, when you do malloc(10 MiB), and then there is memory pressure, 
+    this 10 MiB memory region is backed by the default pager, whose job then is it to provide the backing store for this.
+    <tschwinge>: This is what commonly would be known as a swap partition.
+    <tschwinge>: And this is also the way tmpfs works (as I understand it).
+    <tschwinge>: malloc(10 MiB) can also be mmap(MAP_ANONYMOUS, 10 MIB); that's the same, essentially.
+    <tschwinge>: Now, for ext2fs or any other disk-based file system, this is different:
+    <tschwinge>: The ext2fs translator implements its own backing store, namely it accesses the disk for storing 
+    changed file content, or to read in data from disk if a new file is opened.
+    
+    ...
+
+    (10:36:14) mcsim: who else uses defpager besides tmpfs and kernel?
+    (10:36:27) braunr: normally, nothing directly
+    (10:37:04) mcsim: than why tmpfs should use defpager?
+    (10:37:22) braunr: it's its backend
+    (10:37:28) braunr: backign store rather
+    (10:37:38) braunr: the backing store of most file systems are partitions
+    (10:37:44) braunr: tmpfs has none, it uses the swap space
+    (10:39:31) mcsim: if we allocate memory for tmpfs using vm_allocate, will it be able to use swap partition?
+    (10:39:56) braunr: it should
+    (10:40:27) braunr: vm_allocate just maps anonymous memory
+    (10:41:27) braunr: anonymous memory uses swap space as its backing store too
+    (10:43:47) braunr: but be aware that this part of the vm system is known to have deficiencies
+    (10:44:14) braunr: which is why all mach based implementations have rewritten their default pager
+    (10:45:11) mcsim: what kind of deficiencies?
+    (10:45:16) braunr: bugs
+    (10:45:39) braunr: and design issues, making anonymous memory fragmentation horrible
+    
+    ...
+
+    (15:23:33) antrik: mcsim: vm_allocate doesn't return a memory object; so it can't be passed to clients for mmap()
+    (15:50:37) mcsim: antrik: I use vm_allocate in pager_read_page
+    (15:54:43) antrik: mcsim: well, that means that you have to actually implement a pager yourself
+    (15:56:10) antrik: also, when the kernel asks the pager to write back some pages, it expects the memory to become free.
+    if you are "paging" to ordinary anonymous memory, this doesn't happen; so I expect it to have a very bad effect
+    on system performance
+    (15:56:54) antrik: both can be avoided by just passing a real anonymous memory object, i.e. one provided by the defpager
+    (15:57:07) antrik: only problem is that the current defpager implementation can't really handle that...