From ead87f1165cc1ff5fb809ec11f82866f02bca810 Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Sat, 30 Nov 2019 17:56:08 -0800 Subject: [PATCH] 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@cmpxchg.org Signed-off-by: Johannes Weiner Acked-by: Chris Down Acked-by: Vlastimil Babka Acked-by: David Hildenbrand Acked-by: Michal Hocko Acked-by: Alexey Dobriyan Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin --- kernel/sysctl.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sysctl.c b/kernel/sysctl.c index b6f2f35d0bcf..70665934d53e 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1466,7 +1466,7 @@ 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 = SYSCTL_ONE, .extra2 = &four,