linux-kernel - Re: [RFC] [PATCH] drop

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Date:	Wed, 27 Apr 2011 10:14:53 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: 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@...morbit.com
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