On Sun, 15 Jul 2007, Rafael J. Wysocki wrote:
On Saturday, 14 July 2007 23:34, david@xxxxxxx wrote:
On Sat, 14 Jul 2007, Rafael J. Wysocki wrote:
On Saturday, 14 July 2007 09:51, david@xxxxxxx wrote:
On Fri, 13 Jul 2007, Rafael J. Wysocki wrote:
Ok, now we need a data channel from the old kernel to the hibernate
kernel, to the restore kernel. and the messier the memory layout the
larger this data channel needs to be (hmm, what's the status on the memory
defrag patches being proposed?) if this list can be made small enough it
would work to just have the old kernel put the data in a known location in
ram, and let the other two parts find it (in ram for the hibernate kernel,
in the hibernate image for the wakeup kernel).
I think the hibernation kernel should mmap() the "old" kernel's (and it's
processes') memory available for saving, so that the image-saving process
can read its contents from the original locations.
but I'll bet that not all kernels keep the info in the same place (and
probably not even in the same format). I'm suggesting that a standard be
defined for the format of the data and the location of a pointer to it
that will be maintained across kernel versions.
Yes.
The image-saving kernel needs to have access to the hibernated kernel's
pages data, plus some additional information that should be passed in a
standard format.
but per stable-abi-nonsense the internal structure of the kernel's pages
data isn't an abi. so instead of figuring this out by pokeing around in
the memory of the old kernel and deciding what should be saved and what
shouldn't, the old kernel (which understands the memory structure) should
create a simple map of what should be backed up (either a bitmap or an
extent-style map, depending on how many holes there are expected to be)
and then provide that map to the new kernel. the new kernel (or more
precisely it's userspace) reads the pages it's told to read and writes
them somewhere.
That's reasonable, but the "old" kernel can only do this after handling the
shut down/quiescing of devices, when there is 100% guarantee that the memory
contents will not change.
Ok, that makes sense. and since part of what's being passed along here is
what ram is free as far as the outgoing kernel is concerned, this is
useful info for the new kernel for other situations, not just for the
hibernate operation, so this is probably a reasonable modification to the
kexec call in any case (although a crash-dump kernel may decide not to
trust this info and save everything, it's still useful to know what the
outgoing kernel considers free)
since people are complaining about the amount of ram that a kexec kernel
would take up I'm assuiming it's somethingmore complex then just a bitmap
of all possible pages.
No, it's just bitmaps, AFAICS, and the complaints are a bit overstated, IMO. ;-)
1 bit for each 4k means 1m bits for 4g of ram, or 128k of bitmaps, growing
up to 1m of ram used for 32G of ram in the system. I guess this isn't bad
as long as it doesn't need to be contiguous for the new kernel to access
it.
ok, that makes it a pretty trivial thing to work with. I just need to
learn how to find the bitmaps.
They are created on the fly before the hibernation. The format is described in
kernel/power/snapshot.c .
I'll look through this file, but the format of this is an abi/api to the
userspace and should be documented outside of the code.
Nope. The user space need not know anything about the image contents.
The current implementation of the user space tools use the knowledge of the
image header format, which is given by 'struct swsusp_info', defined in
kernel/power/power.h .
there are a couple factors here.
1. this needs to remain the same across different kernel versions.
2. this may or may not be created by userspace tools
both of these tend to imply that this is an interface to the world and
needs to be documented and stable.
David Lang
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