This is a note to let you know that I've just added the patch titled
kernel: sysctl: make drop_caches write-only
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kernel-sysctl-make-drop_caches-write-only.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 0fb25d4c4d078691af8d938c1fce36c0306885e6
Author: Johannes Weiner <hannes@xxxxxxxxxxx>
Date: Sat Nov 30 17:56:08 2019 -0800
kernel: sysctl: make drop_caches write-only
[ Upstream commit 204cb79ad42f015312a5bbd7012d09c93d9b46fb ]
Currently, the drop_caches proc file and sysctl read back the last value
written, suggesting this is somehow a stateful setting instead of a
one-time command. Make it write-only, like e.g. compact_memory.
While mitigating a VM problem at scale in our fleet, there was confusion
about whether writing to this file will permanently switch the kernel into
a non-caching mode. This influences the decision making in a tense
situation, where tens of people are trying to fix tens of thousands of
affected machines: Do we need a rollback strategy? What are the
performance implications of operating in a non-caching state for several
days? It also caused confusion when the kernel team said we may need to
write the file several times to make sure it's effective ("But it already
reads back 3?").
Link: http://lkml.kernel.org/r/20191031221602.9375-1-hannes@xxxxxxxxxxx
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Chris Down <chris@xxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 24c7fe8608d0..c2dddd335d06 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1357,7 +1357,7 @@ static struct ctl_table vm_table[] = {
.procname = "drop_caches",
.data = &sysctl_drop_caches,
.maxlen = sizeof(int),
- .mode = 0644,
+ .mode = 0200,
.proc_handler = drop_caches_sysctl_handler,
.extra1 = &one,
.extra2 = &four,
