On Sun, 15 Jul 2007, Rafael J. Wysocki wrote:
Hi,
Since many alternative approaches to hibernation are now being considered and
discussed, I thought it might be a good idea to list some things that in my not
so humble opinion should be taken care of by any hibernation framework. They
are listed below, not in any particular order, because I think they all are
important. Still, I might have forgotten something, so everyone with
experience in implementing hibernation, especially Pavel and Nigel, please
check if the list is complete.
(1) Filesystems mounted before the hibernation are untouchable
When there's a memory snapshot, either in the form of a hibernation image,
or in the form of the "old" kernel and processes available to the "new"
kexeced kernel responsible for saving their memory, the filesystems mounted
before the hibernation should not be accessed, even for reading, because
that would cause their on-disk state to be inconsistent with the snapshot
and might lead to a filesystem corruption.
AFAIK this is only the case with ext3, all other filesystems could be
accessed read-only safely
this is arguably a bug with ext3 (and has been discussed as such), but
right now the ext3 team has decided not to change this bahavior so
hibernate needs to work around it. but don't mistake a work-around for a
single (admittedly very popular) filesystem with a hard and fast
directive.
(2) Swap space in use before the hibernation must be handled with care
If swap space is used for saving the memory snapshot, the snapshot-saving
application (or kernel) must be careful enough not to overwrite swap pages
that contain valid memory contents stored in there before the hibernation.
true, in fact, given that many distros and live-CD's autodetect swap
partitions and consider them fair game, I would argue that the best thing
to do would be to have the main system free up it's swap partitions before
going into hibernation.
however, this could be a decision of the particular hibernate routines.
for the kexec approach the mapping of what swap pages are in use is one
more chunk of data that needs to be assembled and made available through a
defined interface.
(4) The user should be able to limit the size of a hibernation image
There are a couple of reasons of that. For example, the storage space
used for saving the image may be smaller than the entire RAM or the user
may want the image to be saved quickier.
it may make sense for this to be split into hard and soft limits.
if you try to save more then the storage space can hold you cannot
continue, but if you are just a little over the arbatrary size limit that
was set to make things fast you are better off saving things as-is then
punting, going back to the system, trying to free more ram, and trying a
hibernate again.
with the kexec approach the enforcment of these limits is also split into
two sections.
when the hibernate command is given in the main kernel, it's userspace
needs to follow some policy to decide how much (if any) memory to free.
