[PATCH] drop-pagecache

Add /proc/sys/vm/drop_caches.  When written to, this will cause the kernel to
discard as much pagecache and/or reclaimable slab objects as it can.  THis
operation requires root permissions.

It won't drop dirty data, so the user should run `sync' first.

Caveats:

a) Holds inode_lock for exorbitant amounts of time.

b) Needs to be taught about NUMA nodes: propagate these all the way through
   so the discarding can be controlled on a per-node basis.

This is a debugging feature: useful for getting consistent results between
filesystem benchmarks.  We could possibly put it under a config option, but
it's less than 300 bytes.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bc01fff3aa01..83c651f25188 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1036,5 +1036,12 @@ int in_gate_area_no_task(unsigned long addr);
 /* /proc/<pid>/oom_adj set to -17 protects from the oom-killer */
 #define OOM_DISABLE -17
 
+int drop_caches_sysctl_handler(struct ctl_table *, int, struct file *,
+                                        void __user *, size_t *, loff_t *);
+int shrink_slab(unsigned long scanned, gfp_t gfp_mask,
+                        unsigned long lru_pages);
+void drop_pagecache(void);
+void drop_slab(void);
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index a9b80fc7f0f3..4cd267fe87ec 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -180,6 +180,7 @@ enum
         VM_VFS_CACHE_PRESSURE=26, /* dcache/icache reclaim pressure */
         VM_LEGACY_VA_LAYOUT=27, /* legacy/compatibility virtual address space layout */
         VM_SWAP_TOKEN_TIMEOUT=28, /* default time for token time out */
+        VM_DROP_PAGECACHE=29,   /* int: nuke lots of pagecache */
 };