diff mbox series

[4.9,151/171] kernel: sysctl: make drop_caches write-only

Message ID 20200102220607.897153673@linuxfoundation.org
State New
Headers show
Series None | expand

Commit Message

Greg KH Jan. 2, 2020, 10:08 p.m. UTC 
From: Johannes Weiner <hannes@cmpxchg.org>

[ 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 <hannes@cmpxchg.org>
Acked-by: Chris Down <chris@chrisdown.name>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sysctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
diff mbox series

Patch

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,