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Subject/proc/sys/vm/drop_caches: add error handling

Document that drop_caches is unsafe, and add error checking so that it
bails out on invalid inputs. [Note that this was triggered by Android
trying to use it in production, and incidentally writing invalid
value...]

Signed-off-by: Pavel Machek <pavel@suse.cz>

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 3197fc8..b90f050 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -132,6 +132,10 @@ To free pagecache, dentries and inodes:
As this is a non-destructive operation and dirty objects are not freeable, the
user should run `sync' first.

+Note that calling this causes some serious latencies, and that this is
+a debug feature; it should not be used for production as it does not
+contain neccessary locking to guarantee safe operation.
+
==============================================================

hugepages_treat_as_movable
diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index 3e5637f..765dec3 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -9,8 +9,9 @@
#include <linux/sysctl.h>
#include <linux/gfp.h>

-/* A global variable is a bit ugly, but it keeps the code simple */
-int sysctl_drop_caches;
+/* A global variable is a bit ugly, and has locking problems,
+ but it keeps the code simple */
+unsigned int sysctl_drop_caches;

static void drop_pagecache_sb(struct super_block *sb)
{
@@ -65,12 +66,17 @@ static void drop_slab(void)
int drop_caches_sysctl_handler(ctl_table *table, int write,
struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
{
- proc_dointvec_minmax(table, write, file, buffer, length, ppos);
- if (write) {
- if (sysctl_drop_caches & 1)
- drop_pagecache();
- if (sysctl_drop_caches & 2)
- drop_slab();
- }
- return 0;
+ int res;
+ res = proc_dointvec_minmax(table, write, file, buffer, length, ppos);
+ if (res)
+ return res;
+ if (!write)
+ return res;
+ if (sysctl_drop_caches & ~3)
+ return -EINVAL;
+ if (sysctl_drop_caches & 1)
+ drop_pagecache();
+ if (sysctl_drop_caches & 2)
+ drop_slab();
+ return res;
}


--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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