The question of whether the i386 architecture should move to using 4K
kernel stacks by default has been raised a few times; LWN last
covered the 4K stack issue in
September. Adrian Bunk has started the discussion anew with
this proposal that the -mm tree
go to 4K stacks (only) now, with an eye toward changing the mainline for
2.6.16.
Most of the technical issues have not changed since September, so those
arguments will not be repeated here. It is worth noting that layered block
devices and filesystems have mostly been fixed. In past kernels, highly
stacked devices (think of a combination of RAID, encryption, and network
filesystems) could end up with very long call chains in the kernel, and, as
a result, overflow the kernel stack. Most of these calls have since been
serialized, so block-layer stacking should not be a problem.
The issue that remains is NDISwrapper, the glue layer which allows Windows
NDIS drivers to be loaded into a Linux kernel. Windows runs with a much
larger kernel stack size, so NDIS driver writers have no reason to be as
careful about stack usage. And, of course, these drivers cannot be fixed
to work properly with Linux. Some have argued that breaking NDISwrapper is
not a possibility, since many users rely upon it to make their wireless
network adapters work. But patience with this line of thought is running
thin, as can be seen in this outburst from
Dave Jones:
If we continue down this path, we'll have no native wireless
drivers for Linux. The answer is not to complain to linux-kernel
for breaking ndiswrapper, but complain to the vendors for not
releasing specifications for native drivers to be written.
The good news is that the wireless situation is
not as bad as one might think. There is documentation for Broadcom
chips available now, and a
Broadcom driver is in the works. There is also an
Atheros driver which is "nearly done." Once these drivers are complete
and joined with the Intel drivers already in the mainline, Linux will have
much better support for wireless devices, and far fewer systems will have
any reason to use NDISwrapper.
There are a number of reasons for going with the 4K stack mode, including
better performance and higher reliability. Some distributions (e.g. Fedora
Core and RHEL) have been shipping 4K kernels for a while now. So, while
nobody has committed to moving the mainline (or -mm) toward 4K-only yet,
chances are improving that it will happen sometime in the not-too-distant
future.
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