[patch]: do not subtract NR_FILE_MAPPED in minimum_image_size()

[Rainer Fiebig <jrf@mailbox.org>]

Hi!

s2disk/s2both may fail unnecessarily and erratically if NR_FILE_MAPPED
is high - for instance when using VMs with VirtualBox and perhaps VMware
Player. In those situations s2disk becomes unreliable and therefore
unusable.

A typical scenario is: user issues a s2disk and it fails. User issues a
second s2disk immediately after that and it succeeds. And user wonders why.

The problem is caused by minimum_image_size() in snapshot.c. The value
it returns is roughly 100 % too high because NR_FILE_MAPPED is
subtracted in its calculation which is imo wrong. Eventually the number
of preallocated image pages is falsely too low.

This doesn't matter as long as NR_FILE_MAPPED-values are in a normal
range or in 32bit-environments as the code allows for allocation of
additional pages from highmem.

But with the high values generated by VirtualBox-VMs (a 2-GB-VM causes
NR_FILE_MAPPED go up by 2 GB) it may lead to failure in 64bit-systems.

Not subtracting NR_FILE_MAPPED in minimum_image_size() solves the problem.

I've done at least hundreds of successful s2both/s2disk now on an x86_64
system (with and without VirtualBox) which gives me some confidence that
this is right. It has turned s2disk/s2both from unusable into 100% reliable.

As I don't have 32bit-equipment, I could not test it in such
environment, though.

The problem has also been discussed in a bug-thread which I closed
myself because it obviously led to nowhere
(https://bugzilla.kernel.org/show_bug.cgi?id=97201).

Thoughts?

Rainer Fiebig

___

--- /kernel/power/snapshot.c
+++ /kernel/power/snapshot.c
@@ -1641,8 +1641,7 @@
  * [number of saveable pages] - [number of pages that can be freed in
theory]
  *
  * where the second term is the sum of (1) reclaimable slab pages, (2)
active
- * and (3) inactive anonymous pages, (4) active and (5) inactive file
pages,
- * minus mapped file pages.
+ * and (3) inactive anonymous pages, (4) active and (5) inactive file
pages.
  */
 static unsigned long minimum_image_size(unsigned long saveable)
 {
@@ -1652,8 +1651,7 @@
 		+ global_node_page_state(NR_ACTIVE_ANON)
 		+ global_node_page_state(NR_INACTIVE_ANON)
 		+ global_node_page_state(NR_ACTIVE_FILE)
-		+ global_node_page_state(NR_INACTIVE_FILE)
-		- global_node_page_state(NR_FILE_MAPPED);
+		+ global_node_page_state(NR_INACTIVE_FILE);

 	return saveable <= size ? 0 : saveable - size;
 }