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SubjectHibernation considerations

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.

(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.

(3) There are memory regions that must not be saved or restored

Some memory regions contain data that shouldn't be overwritten during the
restore, because that might lead to the system not working correctly
afterwards. Also, on some systems there are valid 'struct pages'
structures that in fact corresond to memory holes and we should not attempt
to save those pages.

(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.

(5) Hibernation should be transparent from the applications' point of view

Generally, applications should not notice that hibernation took place.
[Note that I don't regard all processes as applications and I think that
there may be processes which need to handle the hibernation in a special
way.] Ideally, for example, if some audio is being played when a
hibernation starts, the audio player should be able to continue playing the
same audio after the restore from the point in which it has been
interrupted by the hibernation. Also, the CPU affinities and similar
settings requested by the applications before a hibernation should be
binding after the restore.

(6) State of devices from before hibernation should be restored, if possible

If possible, during a restore devices should be brought back to the same
state in which they were before the corresponding hibernation. Of course
in some situations it might be impossible to do that (eg. the user
connected the hibernated system to a different IP subnet and then
restored), but as a general rule, we should do our best to restore the
state of devices, which is directly related to point (5) above.

(7) On ACPI systems special platform-related actions have to be carried out at
the right points, so that the platform works correctly after the restore

The ACPI specification requires us to invoke some global ACPI methods
during the hibernation and during the restore. Moreover, the ordering of
code related to these ACPI methods may not be arbitrary (eg. some of
them have to be executed after devices are put into low power states etc.).

(8) Hibernation and restore should not be too slow

In my opinion, if more than one minute is needed to hibernate the system
with the help of certain hibernation framework, then this framework is not
very useful in practice. It might be useful to perform some special tasks
(eg. moving a server to another place without taking it down), but it is
not very useful, for example, to notebook users.

(9) Hibernation framework should not be too difficult to set up

It follows from my experience that if the users are required to do too much
work to set up a hibernation framework, they will not use it as long as
there are simpler alternatives (some of them will not use hibernation at
all if it's too difficult to get to work). On the other hand, if the users
are provided with a working hibernation framework by their distribution
and they find it useful, they are not likely to use kernels if
t's too difficult to replace the distribution kernel with a generic one due
to the hibernation framework's requirements.

All of the existing hibernation frameworks have been written with the above
points in mind and that's why they are what they are. In particular, the
existence of the tasks freezer, hated by some people to the point of insanity,
follows directly from points (1), (4) and (5).

In my opinion any hibernation framework that doesn't take the above
requirements into account in any way will be a failure. Moreover, the existing
frameworks fail to follow some of them too, so I consider all of these
frameworks as a work in progress. For this reason, I will much more appreciate
ideas allowing us to improve the existing frameworks in a more or less
evolutionary way, then attempts to replace them all with something entirely


"Premature optimization is the root of all evil." - Donald Knuth
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