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Subject[PATCH v2] Make vm_max_readahead configurable at run-time
Date
Incorporated coding-style changes, better changelog and documentation as
suggested by Andrew and Balbir.

Thanks
Nikanth

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.

Also filesystems which use default_backing_dev_info would also
use this new value, even if they were already mounted.

Currently xfs, btrfs, nilfs, raw, mtd use the default_backing_dev_info.


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_kb * 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_kb * 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_KB 128
+extern unsigned long vm_max_readahead_kb;
+
+int sysctl_vm_max_readahead_kb_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *length, loff_t *ppos);

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 +18,8 @@ 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_KB
+ >> (PAGE_CACHE_SHIFT - 10),
.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_kb = INITIAL_VM_MAX_READAHEAD_KB;
+
+int sysctl_vm_max_readahead_kb_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_kb >> (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_kb,
+ .maxlen = sizeof(vm_max_readahead_kb),
+ .mode = 0644,
+ .proc_handler = sysctl_vm_max_readahead_kb_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 +264,18 @@ The minimum value is 1 (1/1 -> 100%).

==============================================================

+max_readahead_kb:
+
+This file contains the default maximum readahead that would be
+used, when new disks would be added to the system.
+
+Also filesystems which use default_backing_dev_info would also
+use this new value, even if they were already mounted.
+
+xfs, btrfs, nilfs, raw, mtd use the default_backing_dev_info.
+
+==============================================================
+
max_map_count:

This file contains the maximum number of memory map areas a process


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