[RFC][PATCH -mm 6/6] Freezer: Document relationship with memory shrinking

Rafael J. Wysocki
2007-07-09T20:41:28Z

From: Rafael J. Wysocki <rjw@sisk.pl>

One important reason to freeze tasks, which is that we don't want them to
allocate memory after freeing it for the hibernation image, has not been
documented. Fix it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
Documentation/power/freezing-of-tasks.txt | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

Index: linux-2.6.22-rc7/Documentation/power/freezing-of-tasks.txt
===================================================================
--- linux-2.6.22-rc7.orig/Documentation/power/freezing-of-tasks.txt
+++ linux-2.6.22-rc7/Documentation/power/freezing-of-tasks.txt
@@ -88,7 +88,16 @@ hibernation image has been created and b
The majority of these are user space processes, but if any of the kernel threads
may cause something like this to happen, they have to be freezable.

-2. The second reason is to prevent user space processes and some kernel threads
+2. Next, to create the hibernation image we need to free a sufficient amount of
+memory (approximately 50% of available RAM) and we need to do that before
+devices are deactivated, because we generally need them for swapping out. Then,
+after the memory for the image has been freed, we don't want tasks to allocate
+additional memory and we prevent them from doing that by freezing them earlier.
+[Of course, this also means that device drivers should not allocate substantial
+amounts of memory from their .suspend() callbacks before hibernation, but this
+is e separate issue.]
+
+3. The third reason is to prevent user space processes and some kernel threads
from interfering with the suspending and resuming of devices. A user space
process running on a second CPU while we are suspending devices may, for
example, be troublesome and without the freezing of tasks we would need some
@@ -118,7 +127,7 @@ frozen before the driver's .suspend() ca
thawed after the driver's .resume() callback has run, so it won't be accessing
the device while it's suspended.

-3. Another reason for freezing tasks is to prevent user space processes from
+4. Another reason for freezing tasks is to prevent user space processes from
realizing that hibernation (or suspend) operation takes place. Ideally, user
space processes should not notice that such a system-wide operation has occurred
and should continue running without any problems after the restore (or resume