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SubjectRe: kexec reboot code buffer
"Martin J. Bligh" <> writes:

> >> We talked about creating a new zone specifically for DMA32 (ie <4Gb)
> >> for other reasons, but it's not there as yet.
> >
> > Right. And because of that I don't feel bad about asking for a zone
> > that ends at 4GB, as it is a fairly general need in the kernel, even
> > if the rest of the interfaces have a little catching up to do before
> > the can use it. Although with IOMMUs I don't know how much such a
> > DMA32 zone is worth.
> It's probably too late for that sort of thing now in 2.5 though.

Unless there are balancing issues adding a new zone is very trivial.
However any code in that direction will be an additional patch so
because ZONE_NORMAL works for most cases. Alan Cox can have his high
memory case if he wants it.

> > I am fine with memory that is not physically contiguous. The memory
> > I really want the kernel is currently sitting on.....
> Oh, in that case you should have no problem getting it from ZONE_NORMAL,
> especially if you can wake up kswapd and wait for a few seconds.

Nope, kswapd will not free the kernels text segment. So in practice
I can use anything below 4GB.

> > The largest I have heard of is currently is 96MB. Typical is
> Eeek! ;-)

There is even a distribution built to be run completely out of a ramdisk.

> > somewhere between 900K and 6MB. You get some interesting
> > kernel+ramdisk combinations when people are network booting a diskless
> > system. Theoretically I can accommodate a nearly 4GB image, with the
> > current code structure.
> Personnally I don't have a problem setting aside that much space at boot
> time, but it's probably not a good solution for small boxes.
> > So the 4GB instead of 960MB limit, and not pesimizing the kernel for
> > the cases where the new image sits in ram for a while (kexec on panic)
> > is while I modified my code to use high memory.
> Maybe IFF you want to suppork kexec on panic, it should be statically
> reserved at boot time? You don't want to be mucking around in the panic
> path trying to swap out memory, etc. when your kernel is halfway down
> the toilet already ... and that has nothing to do with memory placement,
> it's just a space issue.

As it currently exists kexec happens in two stages sys_kexec_load that
scatters the new image through out memory, with some care so that I can
use memcpy, and some variant of memmove to place the image at it's
final location in memory. This is where all of the memory allocation

Then there is sys_reboot(LINUX_REBOOT_CMD_KEXEC), (or a kernel panic)
which triggers the work. And except for cpu state changes nothing

> > The nasty case comes with highmemory when I am allocating memory on a
> > 32GB NUMA box and am allocating memory on the wrong node. In which
> > case my code needs to allocate 28GB before it starts getting the
> > memory it wants.
> Oh, just do alloc_pages_node(0) (works on non NUMA as well, will just
> fall back). But I can show you a 32Gb SMP box as well ;-) ZONE_NORMAL
> is probably still easiest, and most general.

Right it is not hard to do it just takes a little more work.

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