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
Signed-off-by: Andrea Righi <[email protected]>
---
fs/drop_caches.c | 2 +-
include/linux/fadvise.h | 2 ++
include/linux/mm.h | 2 ++
mm/fadvise.c | 7 +++++++
4 files changed, 12 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..dc9ce98 100644
--- a/include/linux/fadvise.h
+++ b/include/linux/fadvise.h
@@ -13,9 +13,11 @@
#if defined(__s390x__)
#define POSIX_FADV_DONTNEED 6 /* Don't need these pages. */
#define POSIX_FADV_NOREUSE 7 /* Data will be accessed once. */
+#define POSIX_FADV_DONTNEED_FS 8 /* Don't need these filesystem pages. */
#else
#define POSIX_FADV_DONTNEED 4 /* Don't need these pages. */
#define POSIX_FADV_NOREUSE 5 /* Data will be accessed once. */
+#define POSIX_FADV_DONTNEED_FS 6 /* Don't need these filesystem pages. */
#endif
#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..4e31fe1 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 (!current_euid())
+ drop_pagecache_sb(file->f_dentry->d_sb, NULL);
+ else
+ ret = -EPERM;
+ break;
default:
ret = -EINVAL;
}
--
1.7.1
On Wednesday 27 April 2011 16:29:05 Andrea Righi wrote:
> diff --git a/include/linux/fadvise.h b/include/linux/fadvise.h
> index e8e7471..dc9ce98 100644
> --- a/include/linux/fadvise.h
> +++ b/include/linux/fadvise.h
> @@ -13,9 +13,11 @@
> #if defined(__s390x__)
> #define POSIX_FADV_DONTNEED 6 /* Don't need these pages. */
> #define POSIX_FADV_NOREUSE 7 /* Data will be accessed once. */
> +#define POSIX_FADV_DONTNEED_FS 8 /* Don't need these filesystem pages. */
> #else
> #define POSIX_FADV_DONTNEED 4 /* Don't need these pages. */
> #define POSIX_FADV_NOREUSE 5 /* Data will be accessed once. */
> +#define POSIX_FADV_DONTNEED_FS 6 /* Don't need these filesystem pages. */
> #endif
>
> #endif /* FADVISE_H_INCLUDED */
Please don't make the s390 mess worse than it already is here.
I think the best solution would be to assign new values starting
from 8 so they can be common for all architectures.
> @@ -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 (!current_euid())
> + drop_pagecache_sb(file->f_dentry->d_sb, NULL);
> + else
> + ret = -EPERM;
> + break;
I somewhat disagree with keying the capability off the UID value, even
if that is what the sysctl uses. CAP_SYS_ADMIN is not that nice either, but
I think it's better than the UID here.
Aside from these, I like the patch.
Arnd
On Wed, Apr 27, 2011 at 05:00:12PM +0200, Arnd Bergmann wrote:
> On Wednesday 27 April 2011 16:29:05 Andrea Righi wrote:
> > diff --git a/include/linux/fadvise.h b/include/linux/fadvise.h
> > index e8e7471..dc9ce98 100644
> > --- a/include/linux/fadvise.h
> > +++ b/include/linux/fadvise.h
> > @@ -13,9 +13,11 @@
> > #if defined(__s390x__)
> > #define POSIX_FADV_DONTNEED 6 /* Don't need these pages. */
> > #define POSIX_FADV_NOREUSE 7 /* Data will be accessed once. */
> > +#define POSIX_FADV_DONTNEED_FS 8 /* Don't need these filesystem pages. */
> > #else
> > #define POSIX_FADV_DONTNEED 4 /* Don't need these pages. */
> > #define POSIX_FADV_NOREUSE 5 /* Data will be accessed once. */
> > +#define POSIX_FADV_DONTNEED_FS 6 /* Don't need these filesystem pages. */
> > #endif
> >
> > #endif /* FADVISE_H_INCLUDED */
>
> Please don't make the s390 mess worse than it already is here.
> I think the best solution would be to assign new values starting
> from 8 so they can be common for all architectures.
Agreed.
>
> > @@ -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 (!current_euid())
> > + drop_pagecache_sb(file->f_dentry->d_sb, NULL);
> > + else
> > + ret = -EPERM;
> > + break;
>
> I somewhat disagree with keying the capability off the UID value, even
> if that is what the sysctl uses. CAP_SYS_ADMIN is not that nice either, but
> I think it's better than the UID here.
OK, I also prefer CAP_SYS_ADMIN.
>
> Aside from these, I like the patch.
>
> Arnd
I'll wait for other comments and post another version with the changes
you suggested.
Thanks for looking at it.
-Andrea