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	microkernel/mach/external_pager_mechanism.mdwn

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+[[!meta copyright="Copyright © 2008, 2009 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]]."]]"""]]
+
+[[!meta title="mtab"]]
+
+In traditional monolithic system, the kernel keeps track of all mounts; the
+information is available through `/proc/mounts` (on Linux at least), and in a
+very similar form in `/etc/mtab`.
+
+The Hurd on the other hand has a totally
+[[decentralized_file_system|hurd/virtual_file_system]].  There is no single
+entity involved in all mounts.  Rather, only the parent file system to which a
+mountpoint ([[hurd/translator]]) is attached is involved.  As a result, there
+is no central place keeping track of mounts.
+
+As a consequence, there is currently no easy way to obtain a listing of all
+mounted file systems. This also means that commands like `df` can only work on
+explicitly specified mountpoints, instead of displaying the usual listing.
+
+One possible solution to this would be for the translator startup mechanism to
+update the `mtab` on any `mount`/`unmount`, like in traditional systems.
+However, there are some problems with this approach.  Most notably: what to do
+with passive translators, i.e., translators that are not presently running, but
+set up to be started automatically whenever the node is accessed?  Probably
+these should be counted among the mounted filesystems; but how to handle the
+`mtab` updates for a translator that is not started yet?  Generally, being
+centralized and event-based, this is a pretty inelegant, non-hurdish solution.
+
+A more promising approach is to have `mtab` exported by a special translator,
+which gathers the necessary information on demand.  This could work by
+traversing the tree of translators, asking each one for mount points attached
+to it.  (Theoretically, it could also be done by just traversing *all* nodes,
+checking each one for attached translators.  That would be very inefficient,
+though.  Thus a special interface is probably required, that allows asking a
+translator to list mount points only.)
+
+There are also some other issues to keep in mind.  Traversing arbitrary
+translators set by other users can be quite dangerous -- and it's probably not
+very interesting anyways what private filesystems some other user has mounted.
+But what about the global `/etc/mtab`?  Should it list only root-owned
+filesystems?  Or should it create different listings depending on what user
+contacts it?...
+
+That leads to a more generic question: which translators should be actually
+listed?  There are different kinds of translators: ranging from traditional
+filesystems ([[disks|hurd/libdiskfs]] and other actual
+[[stores|hurd/translator/storeio]]), but also purely virtual filesystems like
+[[hurd/translator/ftpfs]] or [[hurd/translator/unionfs]], and even things that
+have very little to do with a traditional filesystem, like a
+[[gzip_translator|hurd/translator/storeio]],
+[[mbox_translator|hurd/translator/mboxfs]],
+[[xml_translator|hurd/translator/xmlfs]], or various device file translators...
+Listing all of these in `/etc/mtab` would be pretty pointless, so some kind of
+classification mechanism is necessary.  By default it probably should list only
+translators that claim to be real filesystems, though alternative views with
+other filtering rules might be desirable.
+
+After taking decisions on the outstanding design questions, the student will
+implement both the actual [[mtab_translator|hurd/translator/mtabfs]], and the
+necessary interface(s) for gathering the data.  It requires getting a good
+understanding of the translator mechanism and Hurd interfaces in general.
+
+Possible mentors: Olaf Buddenhagen (antrik), Carl Fredrik Hammar (cfhammar)
+
+Exercise: Make some improvement to any of the existing Hurd translators.
+Especially those in [hurdextras](http://www.nongnu.org/hurdextras/) are often
+quite rudimentary, and it shouldn't be hard to find something to improve.