Re: [RFC] [PATCH] drop_pagecache syscall

On Tue, Apr 26, 2011 at 11:35:27PM +0200, Andrea Righi wrote:

Introduce sys_drop_pagecache() system call to drop the page cache pages of
a single filesystem.

This new system call takes a file descriptor as argument and drops only
the page cache pages of the file system it references.

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. But 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(), but this solution doesn't scale
very well in some cases).

Why not just add a new posix_fadvise() command? e.g.
POSIX_FADV_DONTNEED_FS. Simpler than adding a new syscall...

This functionality can be used by all the applications that want to have a
better control over the page cache management (for example to immediately drop
pages that for sure will not be reused in the near future, without calling
posix_fadvise() for all the files they've touched), or to provide a more fine
grained debugging feature usable by the filesystem benchmarks.

The system call does not require root privileges and it can be called by any
unprivileged application. For example, we can write a userspace tool to run
something like this:

  $ drop-pagecache /path/file_or_dir

That's a potential DOS vector, I think. Drop the pagecache in a hard
loop on the root fs of a busy server and watch it crawl...

+/*
+ * Drop page cache of a single superblock
+ */
+SYSCALL_DEFINE1(drop_pagecache, int, fd)
+{
+	struct file *file;
+	struct super_block *sb;
+	int fput_needed;
+
+	file = fget_light(fd, &fput_needed);
+	if (!file)
+		return -EBADF;
+	sb = file->f_dentry->d_sb;
+
+	down_read(&sb->s_umount);
+	drop_pagecache_sb(sb, NULL);
+	up_read(&sb->s_umount);
+
+	fput_light(file, fput_needed);
+	return 0;

You're holding an open reference to a file/dir on the fs so it can't
be unmounted from under you. Hence I don't think you need the
s_umount locking.

Cheers,

Dave.
-- 
Dave Chinner
david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org