[PATCH v2] fadvise: introduce POSIX_FADV_DONTNEED_FS

From: Andrea Righi
Date: Wed Apr 27 2011 - 14:14:06 EST


Introduce a new fadvise flag to drop page cache pages of a single
filesystem.

At the moment it is possible to drop page cache pages via
/proc/sys/vm/drop_pagecache or via posix_fadvise(POSIX_FADV_DONTNEED).

The first method drops the whole page cache while the second can be used
to drop page cache pages of a single file descriptor. However, there's
not a simple way to drop all the pages of a filesystem (we could scan
all the file descriptors and use posix_fadvise(POSIX_FADV_DONTNEED), but
this solution obviously doesn't scale well).

This functionality requires root privilege to avoid potential DoS in the
system (i.e., a hard loop of posix_fadvise(POSIX_FADV_DONTNEED_FS) on
the root filesystem).

A practical example:

# ls -lh /mnt/sda/zero /mnt/sdb/zero
-rw-r--r-- 1 root root 16M 2011-04-20 10:20 /mnt/sda/zero
-rw-r--r-- 1 root root 16M 2011-04-20 10:20 /mnt/sdb/zero

$ grep ^Cached /proc/meminfo
Cached: 5660 kB
$ md5sum /mnt/sda/zero /mnt/sdb/zero
2c7ab85a893283e98c931e9511add182 /mnt/sda/zero
2c7ab85a893283e98c931e9511add182 /mnt/sdb/zero
$ grep ^Cached /proc/meminfo
Cached: 38544 kB
$ sudo ./drop-pagecache /mnt/sda/
$ grep ^Cached /proc/meminfo
Cached: 22440 kB
$ sudo ./drop-pagecache /mnt/sdb/
$ grep ^Cached /proc/meminfo
Cached: 5056 kB

A previous RFC about this topic can be found here:
http://marc.info/?l=linux-kernel&m=130385374902114&w=2

ChangeLog (v1 -> v2):

* use the same value for POSIX_FADV_DONTNEED_FS on all architectures
* check CAP_SYS_ADMIN capability instead of checking the EUID value

Signed-off-by: Andrea Righi <andrea@xxxxxxxxxxxxxxx>
---
fs/drop_caches.c | 2 +-
include/linux/fadvise.h | 1 +
include/linux/mm.h | 2 ++
mm/fadvise.c | 7 +++++++
4 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index 98b77c8..59d6caa 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -13,7 +13,7 @@
/* A global variable is a bit ugly, but it keeps the code simple */
int sysctl_drop_caches;

-static void drop_pagecache_sb(struct super_block *sb, void *unused)
+void drop_pagecache_sb(struct super_block *sb, void *unused)
{
struct inode *inode, *toput_inode = NULL;

diff --git a/include/linux/fadvise.h b/include/linux/fadvise.h
index e8e7471..ab39117 100644
--- a/include/linux/fadvise.h
+++ b/include/linux/fadvise.h
@@ -17,5 +17,6 @@
#define POSIX_FADV_DONTNEED 4 /* Don't need these pages. */
#define POSIX_FADV_NOREUSE 5 /* Data will be accessed once. */
#endif
+#define POSIX_FADV_DONTNEED_FS 8 /* Don't need these filesystem pages. */

#endif /* FADVISE_H_INCLUDED */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 692dbae..004cdbc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -21,6 +21,7 @@ struct anon_vma;
struct file_ra_state;
struct user_struct;
struct writeback_control;
+struct super_block;

#ifndef CONFIG_DISCONTIGMEM /* Don't use mapnrs, do it properly */
extern unsigned long max_mapnr;
@@ -1602,6 +1603,7 @@ int in_gate_area_no_mm(unsigned long addr);
#define in_gate_area(mm, addr) ({(void)mm; in_gate_area_no_mm(addr);})
#endif /* __HAVE_ARCH_GATE_AREA */

+void drop_pagecache_sb(struct super_block *sb, void *unused);
int drop_caches_sysctl_handler(struct ctl_table *, int,
void __user *, size_t *, loff_t *);
unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
diff --git a/mm/fadvise.c b/mm/fadvise.c
index 8d723c9..15155e7 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -57,6 +57,7 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
case POSIX_FADV_WILLNEED:
case POSIX_FADV_NOREUSE:
case POSIX_FADV_DONTNEED:
+ case POSIX_FADV_DONTNEED_FS:
/* no bad return value, but ignore advice */
break;
default:
@@ -127,6 +128,12 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
invalidate_mapping_pages(mapping, start_index,
end_index);
break;
+ case POSIX_FADV_DONTNEED_FS:
+ if (capable(CAP_SYS_ADMIN))
+ drop_pagecache_sb(file->f_dentry->d_sb, NULL);
+ else
+ ret = -EPERM;
+ break;
default:
ret = -EINVAL;
}
--
1.7.1

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