[PATCH] Make vm_max_readahead configurable at run-time

[Nikanth Karthikesan <knikanth@suse.de>]

Make vm_max_readahead configurable at run-time. Expose a sysctl knob
in procfs to change it. This would ensure that new disks added would
use this value as their default read_ahead_kb.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

---

Index: linux-2.6/block/blk-core.c
===================================================================
--- linux-2.6.orig/block/blk-core.c
+++ linux-2.6/block/blk-core.c
@@ -499,7 +499,7 @@ struct request_queue *blk_alloc_queue_no
 	q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
 	q->backing_dev_info.unplug_io_data = q;
 	q->backing_dev_info.ra_pages =
-			(VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+			(vm_max_readahead * 1024) / PAGE_CACHE_SIZE;
 	q->backing_dev_info.state = 0;
 	q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
 	q->backing_dev_info.name = "block";
Index: linux-2.6/fs/fuse/inode.c
===================================================================
--- linux-2.6.orig/fs/fuse/inode.c
+++ linux-2.6/fs/fuse/inode.c
@@ -870,7 +870,7 @@ static int fuse_bdi_init(struct fuse_con
 	int err;
 
 	fc->bdi.name = "fuse";
-	fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+	fc->bdi.ra_pages = (vm_max_readahead * 1024) / PAGE_CACHE_SIZE;
 	fc->bdi.unplug_io_fn = default_unplug_io_fn;
 	/* fuse does it's own writeback accounting */
 	fc->bdi.capabilities = BDI_CAP_NO_ACCT_WB;
Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -1188,7 +1188,11 @@ int write_one_page(struct page *page, in
 void task_dirty_inc(struct task_struct *tsk);
 
 /* readahead.c */
-#define VM_MAX_READAHEAD	128	/* kbytes */
+#define INITIAL_VM_MAX_READAHEAD	128	/* kbytes */
+extern unsigned long vm_max_readahead;
+
+int sysctl_vm_max_readahead_handler(struct ctl_table *, int,
+					void __user *, size_t *, loff_t *);
 
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			pgoff_t offset, unsigned long nr_to_read);
Index: linux-2.6/mm/backing-dev.c
===================================================================
--- linux-2.6.orig/mm/backing-dev.c
+++ linux-2.6/mm/backing-dev.c
@@ -18,7 +18,7 @@ EXPORT_SYMBOL(default_unplug_io_fn);
 
 struct backing_dev_info default_backing_dev_info = {
 	.name		= "default",
-	.ra_pages	= VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
+	.ra_pages	= INITIAL_VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
 	.state		= 0,
 	.capabilities	= BDI_CAP_MAP_COPY,
 	.unplug_io_fn	= default_unplug_io_fn,
Index: linux-2.6/mm/readahead.c
===================================================================
--- linux-2.6.orig/mm/readahead.c
+++ linux-2.6/mm/readahead.c
@@ -17,6 +17,19 @@
 #include <linux/pagevec.h>
 #include <linux/pagemap.h>
 
+unsigned long vm_max_readahead = INITIAL_VM_MAX_READAHEAD;
+
+int sysctl_vm_max_readahead_handler(struct ctl_table *table, int write,
+		void __user *buffer, size_t *length, loff_t *ppos)
+{
+	proc_doulongvec_minmax(table, write, buffer, length, ppos);
+
+	default_backing_dev_info.ra_pages =
+			vm_max_readahead >> (PAGE_CACHE_SHIFT - 10);
+
+	return 0;
+}
+
 /*
  * Initialise a struct file's readahead state.  Assumes that the caller has
  * memset *ra to zero.
Index: linux-2.6/kernel/sysctl.c
===================================================================
--- linux-2.6.orig/kernel/sysctl.c
+++ linux-2.6/kernel/sysctl.c
@@ -1273,7 +1273,13 @@ static struct ctl_table vm_table[] = {
 		.extra2		= &one,
 	},
 #endif
-
+	{
+		.procname	= "max_readahead_kb",
+		.data		= &vm_max_readahead,
+		.maxlen		= sizeof(vm_max_readahead),
+		.mode		= 0644,
+		.proc_handler	= sysctl_vm_max_readahead_handler,
+	},
 	{ }
 };
 
Index: linux-2.6/Documentation/sysctl/vm.txt
===================================================================
--- linux-2.6.orig/Documentation/sysctl/vm.txt
+++ linux-2.6/Documentation/sysctl/vm.txt
@@ -31,6 +31,7 @@ Currently, these files are in /proc/sys/
 - laptop_mode
 - legacy_va_layout
 - lowmem_reserve_ratio
+- max_readahead_kb
 - max_map_count
 - memory_failure_early_kill
 - memory_failure_recovery
@@ -263,6 +264,12 @@ The minimum value is 1 (1/1 -> 100%).
 
 ==============================================================
 
+max_readahead_kb:
+
+This file contains the default maximum readahead that would be used.
+
+==============================================================
+
 max_map_count:
 
 This file contains the maximum number of memory map areas a process

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