linux-kernel - [PATCH -mm 1/5] Freezer: Document relationship with memory shrinking

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Date:	Wed, 18 Jul 2007 09:30:44 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>
Subject: [PATCH -mm 1/5] Freezer: Document relationship with memory shrinking

From: Rafael J. Wysocki <rjw@...k.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@...k.pl>
Acked-by: Pavel Machek <pavel@....cz>
---
 Documentation/power/freezing-of-tasks.txt |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

Index: linux-2.6.22-rc6-mm1/Documentation/power/freezing-of-tasks.txt
===================================================================
--- linux-2.6.22-rc6-mm1.orig/Documentation/power/freezing-of-tasks.txt	2007-07-11 20:48:04.000000000 +0200
+++ linux-2.6.22-rc6-mm1/Documentation/power/freezing-of-tasks.txt	2007-07-11 20:50:24.000000000 +0200
@@ -81,7 +81,16 @@ 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
@@ -111,7 +120,7 @@ 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

-
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