[PATCH 3/3] Documentation: add instructions for using 'drop_fs_caches sysctl' sysctl

[Ye Bin]

From: Ye Bin <yebin10@xxxxxxxxxx>

Add instructions for 'drop_fs_caches sysctl' sysctl in 'vm.rst'.

Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/vm.rst | 27 +++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index f48eaa98d22d..4648ac1ac66c 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -36,6 +36,7 @@ Currently, these files are in /proc/sys/vm:
 - dirtytime_expire_seconds
 - dirty_writeback_centisecs
 - drop_caches
+- drop_fs_caches
 - enable_soft_offline
 - extfrag_threshold
 - highmem_is_dirtyable
@@ -268,6 +269,32 @@ used::
 These are informational only.  They do not mean that anything is wrong
 with your system.  To disable them, echo 4 (bit 2) into drop_caches.
 
+drop_fs_caches
+==============
+
+Writing to this will cause the kernel to drop clean for a specific file system
+caches, as well as reclaimable slab objects like dentries and inodes. Once
+dropped, their memory becomes free. Except for specifying the device number for
+a specific file system, everything else is consistent with drop_caches. The
+device number can be viewed through "cat /proc/self/montinfo" or 'lsblk'.
+
+To free pagecache::
+
+	echo "MAJOR:MINOR:1" > /proc/sys/vm/drop_caches
+
+To free reclaimable slab objects (includes dentries and inodes)::
+
+	echo "MAJOR:MINOR:2" > /proc/sys/vm/drop_caches
+
+To free slab objects and pagecache::
+
+	echo "MAJOR:MINOR:3" > /proc/sys/vm/drop_caches
+
+You may see informational messages in your kernel log when this file is
+used::
+
+	echo (1234): drop_fs_caches: MAJOR:MINOR:3
+
 enable_soft_offline
 ===================
 Correctable memory errors are very common on servers. Soft-offline is kernel's
-- 
2.31.1