Andrew Morton's -mm kernel tree now fills the role which might have once
been taken by an odd-numbered development series. We don't have 2.7.x;
instead, new stuff finds its way into 2.6.x-mm. So it can be interesting
to step back, occasionally, and look at what patches are lurking there.
2.6.9-rc3-mm2 contains a full 1213 patches. About half of these come from
trees managed by various subsystem maintainers; seeing what those are
usually requires pulling a separate BitKeeper tree and looking inside.
These trees hold patches which are usually (usually!) relatively small and
maintenance-oriented. The external trees brought into -mm currently
include those dedicated to the ACPI, AGPGART, ALSA, i2c, IDE, IEEE 1394,
input, serial ATA, networking, NTFS, driver core, PCI, USB, and SCSI
Among the other 654 patches in 2.6.9-rc3-mm2 are found:
- A change to how rlimit settings are interpreted; they become
per-process settings, rather than per-thread.
- The sysfs backing store patches
continue to languish in -mm, apparently waiting for a review from some
of the core developers.
- Ingo Molnar's "generic IRQ subsystem" work. These patches, posted on October 2, are a big
reorganization of the interrupt handling code. Over the years, much
of the IRQ code had been copied from one architecture to the next,
leading to a lot of duplicated functions. These patches pull the
generic code out of the architecture subtrees and remove some 3000
lines of code from the kernel.
- Numerous kernel debugger (kgdb) patches continue to live in -mm; as
always, they are unlikely to move into the mainline.
- They get less attention than they used to, but there are still must-fix and should-fix lists in -mm.
- Arjan van de Ven's patch which keeps processes from being able to
overwrite kernel memory via /dev/mem. This patch has been
shipped with Red Hat/Fedora kernels for a while, but is not yet in the
- An extensive set of ext3 patches implementing block reservations. Stephen Tweedie has
recently resumed working on these patches, so they may move forward in
the near future. The ext3 online resizing patch set is also in -mm.
- Mikael Pettersson's performance counters patches.
- The -mm tree continues to be a testing ground for scheduler patches.
It currently contains Peter Williams's Single Priority Array scheduler
(covered briefly here last August).
There is also an extensive set of scheduling domains fixes and a
number of latency-reduction patches from Ingo Molnar's work.
- Ingo Molnar's big kernel semaphore
- A set of PCMCIA patches adding driver model and hotplug support.
- A big DVD+RW support patch, which includes CDRW packet writing
- Support for in-kernel keyrings and their management.
- The CacheFS filesystem.
- The kexec patches, including support for using kexec as a kernel crash
- The reiser4 filesystem and a large number of fixes.
- The modular I/O schedulers patch and
the reworked "completely fair queueing" scheduler.
- The remap_page_range() change
- A security module implementing the BSD "secure levels" mechanism.
Mixed in with these big patches is the usual array of architecture updates,
subsystem fixes, etc.
In other words, -mm is a big patch; it is significantly different from the
mainline kernel. For some developers, it is too far removed; David Miller
recently responded to a request to test
networking changes in -mm this way:
Putting the net stuff into -mm makes debugging of networking
changes harder, as -mm has a ton of experimental stuff in it as
well. -mm frequently makes machines unbootable, and particularly
this is felt on non-x86 platforms such as sparc64 which is where I
do all of my work.
This kind of observation is not new; many developers continued to create
their patches on the 2.4 kernel long after the 2.5 branch opened because
2.5 struck them as being too unstable. When one is trying to shake out
bugs in new code, it is nice to minimize the number of other unrelated,
disruptive changes. That said, -mm continues to be the main staging area
for much of the code going into the mainline, and many developers target it
specifically with their patches. Given the number of bugs found after
patches go into -mm, people are clearly running it as well.
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