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 4.9-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-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit d0bf2ce2b2437bc6413710d245683601a8f88371 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 6af1ac551ea3..34449ec0689d 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1398,7 +1398,7 @@ 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,