linux-kernel - Re: [V2 PATCH] tmpfs: add fallocate support

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Date:	Tue, 22 Nov 2011 14:06:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cong Wang <amwang@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kay Sievers <kay.sievers@...y.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: [V2 PATCH] tmpfs: add fallocate support

On Fri, 18 Nov 2011 18:39:50 +0800
Cong Wang <amwang@...hat.com> wrote:

> It seems that systemd needs tmpfs to support fallocate,
> see http://lkml.org/lkml/2011/10/20/275. This patch adds
> fallocate support to tmpfs.
> 
> As we already have shmem_truncate_range(), it is also easy
> to add FALLOC_FL_PUNCH_HOLE support too.
> 
>
> ...
>
> +static long shmem_fallocate(struct file *file, int mode,
> +				loff_t offset, loff_t len)
> +{
> +	struct inode *inode = file->f_path.dentry->d_inode;
> +	pgoff_t start = offset >> PAGE_CACHE_SHIFT;
> +	pgoff_t end = DIV_ROUND_UP((offset + len), PAGE_CACHE_SIZE);
> +	pgoff_t index = start;
> +	loff_t i_size = i_size_read(inode);
> +	struct page *page = NULL;
> +	int ret = 0;
> +
> +	mutex_lock(&inode->i_mutex);

It would be saner and less racy-looking to read i_size _after_ taking
i_mutex.

And if you do that, there's no need to use i_size_read() - just a plain
old

	i_size = inode->i_size;

is OK.


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