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    Subject[PATCH AUTOSEL 4.19 78/79] kernel: sysctl: make drop_caches write-only
    Date
    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 --git a/kernel/sysctl.c b/kernel/sysctl.c
    index f8576509c7bef..4c4fd4339d330 100644
    --- a/kernel/sysctl.c
    +++ b/kernel/sysctl.c
    @@ -1411,7 +1411,7 @@ 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,
    --
    2.20.1
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