linux-kernel - Re: [PATCH 2/4] vfs: Protect write paths by sb_start_write

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Message-Id: <FA7A1011-4312-4050-8AD5-C6060CFF3A55@dilger.ca>
Date:	Thu, 12 Jan 2012 12:56:01 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, xfs@....sgi.com,
	Eric Sandeen <sandeen@...deen.net>,
	Dave Chinner <dchinner@...hat.com>,
	Surbhi Palande <csurbhi@...il.com>,
	Kamal Mostafa <kamal@...onical.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/4] vfs: Protect write paths by sb_start_write - sb_end_write

On 2012-01-11, at 6:20 PM, Jan Kara wrote:
> There are three entry points which dirty pages in a filesystem.  mmap (handled
> by block_page_mkwrite()), buffered write (handled by
> __generic_file_aio_write()), and truncate (it can dirty last partial page -
> handled by do_truncate()). Protect these places with sb_start_write() and
> sb_end_write().
> 
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
> fs/buffer.c  |   18 ++----------------
> fs/open.c    |    6 ++++++
> mm/filemap.c |    3 ++-
> 3 files changed, 10 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 19d8eb7..8519405 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2371,18 +2371,7 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,

The comment for __block_page_mkwrite() needs to be updated to reference
sb_start_write() and sb_end_write() instead of vfs_check_frozen().

Cheers, Andreas

> 	if (unlikely(ret < 0))
> 		goto out_unlock;
> -	/*
> -	 * Freezing in progress? We check after the page is marked dirty and
> -	 * with page lock held so if the test here fails, we are sure freezing
> -	 * code will wait during syncing until the page fault is done - at that
> -	 * point page will be dirty and unlocked so freezing code will write it
> -	 * and writeprotect it again.
> -	 */
> 	set_page_dirty(page);
> -	if (inode->i_sb->s_frozen != SB_UNFROZEN) {
> -		ret = -EAGAIN;
> -		goto out_unlock;
> -	}
> 	wait_on_page_writeback(page);
> 	return 0;
> out_unlock:
> @@ -2397,12 +2386,9 @@ int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> 	int ret;
> 	struct super_block *sb = vma->vm_file->f_path.dentry->d_inode->i_sb;
> 
> -	/*
> -	 * This check is racy but catches the common case. The check in
> -	 * __block_page_mkwrite() is reliable.
> -	 */
> -	vfs_check_frozen(sb, SB_FREEZE_WRITE);
> +	sb_start_write(sb);
> 	ret = __block_page_mkwrite(vma, vmf, get_block);
> +	sb_end_write(sb);
> 	return block_page_mkwrite_return(ret);
> }
> EXPORT_SYMBOL(block_page_mkwrite);
> diff --git a/fs/open.c b/fs/open.c
> index 22c41b5..ee17c90 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -55,8 +55,14 @@ int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
> 	if (ret)
> 		newattrs.ia_valid |= ret | ATTR_FORCE;
> 
> +	/*
> +	 * Truncate can dirty last partial page so we need protection against
> +	 * filesystem freezing.
> +	 */
> 	mutex_lock(&dentry->d_inode->i_mutex);
> +	sb_start_write(dentry->d_sb);
> 	ret = notify_change(dentry, &newattrs);
> +	sb_end_write(dentry->d_sb);
> 	mutex_unlock(&dentry->d_inode->i_mutex);
> 	return ret;
> }
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c0018f2..6566c73 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2529,7 +2529,7 @@ ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
> 	count = ocount;
> 	pos = *ppos;
> 
> -	vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
> +	sb_start_write(inode->i_sb);
> 
> 	/* We can write back this queue in page reclaim */
> 	current->backing_dev_info = mapping->backing_dev_info;
> @@ -2601,6 +2601,7 @@ ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
> 				pos, ppos, count, written);
> 	}
> out:
> +	sb_end_write(inode->i_sb);
> 	current->backing_dev_info = NULL;
> 	return written ? written : err;
> }
> -- 
> 1.7.1
> 
> --
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