Commit 5509a5d2 authored by Dave Hansen's avatar Dave Hansen Committed by Linus Torvalds

drop_caches: add some documentation and info message

There is plenty of anecdotal evidence and a load of blog posts
suggesting that using "drop_caches" periodically keeps your system
running in "tip top shape".  Perhaps adding some kernel documentation
will increase the amount of accurate data on its use.

If we are not shrinking caches effectively, then we have real bugs.
Using drop_caches will simply mask the bugs and make them harder to
find, but certainly does not fix them, nor is it an appropriate
"workaround" to limit the size of the caches.  On the contrary, there
have been bug reports on issues that turned out to be misguided use of
cache dropping.

Dropping caches is a very drastic and disruptive operation that is good
for debugging and running tests, but if it creates bug reports from
production use, kernel developers should be aware of its use.

Add a bit more documentation about it, a syslog message to track down
abusers, and vmstat drop counters to help analyze problem reports.

[ checkpatch fixes]
[ add runtime suppression control]
Signed-off-by: default avatarDave Hansen <>
Signed-off-by: default avatarMichal Hocko <>
Acked-by: default avatarKOSAKI Motohiro <>
Acked-by: default avatarKAMEZAWA Hiroyuki <>
Signed-off-by: default avatarJohannes Weiner <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 67f9fd91
......@@ -175,18 +175,39 @@ Setting this to zero disables periodic writeback altogether.
Writing to this will cause the kernel to drop clean caches, dentries and
inodes from memory, causing that memory to become free.
Writing to this will cause the kernel to drop clean caches, as well as
reclaimable slab objects like dentries and inodes. Once dropped, their
memory becomes free.
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
To free reclaimable slab objects (includes dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
To free slab objects and pagecache:
echo 3 > /proc/sys/vm/drop_caches
As this is a non-destructive operation and dirty objects are not freeable, the
user should run `sync' first.
This is a non-destructive operation and will not free any dirty objects.
To increase the number of objects freed by this operation, the user may run
`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the
number of dirty objects on the system and create more candidates to be
This file is not a means to control the growth of the various kernel caches
(inodes, dentries, pagecache, etc...) These objects are automatically
reclaimed by the kernel when memory is needed elsewhere on the system.
Use of this file can cause performance problems. Since it discards cached
objects, it may cost a significant amount of I/O and CPU to recreate the
dropped objects, especially if they were under heavy use. Because of this,
use outside of a testing or debugging environment is not recommended.
You may see informational messages in your kernel log when this file is
cat (1234): drop_caches: 3
These are informational only. They do not mean that anything is wrong
with your system. To disable them, echo 4 (bit 3) into drop_caches.
......@@ -59,10 +59,22 @@ int drop_caches_sysctl_handler(ctl_table *table, int write,
if (ret)
return ret;
if (write) {
if (sysctl_drop_caches & 1)
static int stfu;
if (sysctl_drop_caches & 1) {
iterate_supers(drop_pagecache_sb, NULL);
if (sysctl_drop_caches & 2)
if (sysctl_drop_caches & 2) {
if (!stfu) {
pr_info("%s (%d): drop_caches: %d\n",
current->comm, task_pid_nr(current),
stfu |= sysctl_drop_caches & 4;
return 0;
......@@ -37,6 +37,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
......@@ -123,7 +123,7 @@ static int __maybe_unused neg_one = -1;
static int zero;
static int __maybe_unused one = 1;
static int __maybe_unused two = 2;
static int __maybe_unused three = 3;
static int __maybe_unused four = 4;
static unsigned long one_ul = 1;
static int one_hundred = 100;
......@@ -1264,7 +1264,7 @@ static struct ctl_table vm_table[] = {
.mode = 0644,
.proc_handler = drop_caches_sysctl_handler,
.extra1 = &one,
.extra2 = &three,
.extra2 = &four,
......@@ -813,6 +813,9 @@ const char * const vmstat_text[] = {
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