Date: Fri, 10 Nov 2006 10:47:24 -0800
From: Andrew Morton <akpm@osdl.org>
To: Kirill Korotaev <dev@openvz.org>
Cc: ext2-devel@lists.sourceforge.net,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       devel@openvz.org, Andrey Savochkin <saw@sw.ru>, sct@redhat.com,
       adilger@clusterfs.com
Subject: Re: [PATCH] retries in ext3_prepare_write() violate ordering
 requirements
Message-Id: <20061110104724.27c30c53.akpm@osdl.org>
In-Reply-To: <455492F9.7080300@openvz.org>
References: <455492F9.7080300@openvz.org>
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On Fri, 10 Nov 2006 17:55:53 +0300
Kirill Korotaev <dev@openvz.org> wrote:

> in journal=ordered or journal=data mode retry in ext3_prepare_write()
> breaks the requirements of journaling of data with respect to metadata.
> The fix is to call commit_write to commit allocated zero blocks before
> retry.
> 

How was this problem detected? (ie: why was block_prepare_write() failing?)

How was the patch tested?

Was nobh-mode also tested?

> 
> --- ./fs/ext3/inode.c.ext3pw	2006-11-08 17:44:14.000000000 +0300
> +++ ./fs/ext3/inode.c	2006-11-08 17:48:59.000000000 +0300
> @@ -1148,37 +1148,89 @@ static int do_journal_get_write_access(h
>  	return ext3_journal_get_write_access(handle, bh);
>  }
>  
> +/*
> + * The idea of this helper function is following:
> + * if prepare_write has allocated some blocks, but not all of them, the
> + * transaction must include the content of the newly allocated blocks.
> + * This content is expected to be set to zeroes by block_prepare_write().
> + * 2006/10/14  SAW
> + */
> +static int ext3_prepare_failure(struct file *file, struct page *page,
> +				unsigned from, unsigned to)
> +{
> +	struct address_space *mapping;
> +	struct buffer_head *bh, *head, *next;
> +	unsigned block_start, block_end;
> +	unsigned blocksize;
> +
> +	mapping = page->mapping;
> +	if (ext3_should_writeback_data(mapping->host)) {
> +		/* optimization: no constraints about data */
> +skip:
> +		ext3_journal_stop(ext3_journal_current_handle());
> +		return 0;

Should this be `return ext3_journal_stop(...);'?

> +	}
> +
> +	head = page_buffers(page);
> +	blocksize = head->b_size;
> +	for (	bh = head, block_start = 0;
> +		bh != head || !block_start;
> +	    	block_start = block_end, bh = next)
> +	{
> +		next = bh->b_this_page;
> +		block_end = block_start + blocksize;
> +		if (block_end <= from)
> +			continue;
> +		if (block_start >= to) {
> +			block_start = to;
> +			break;
> +		}
> +		if (!buffer_mapped(bh))
> +			break;

What is the significance of buffer_mapped() here?  Outside EOF or into a
hole?  If so, then block_start >= to, and we can't get here??

> +	}
> +	if (block_start <= from)
> +		goto skip;
> +
> +	/* commit allocated and zeroed buffers */
> +	return mapping->a_ops->commit_write(file, page, from, block_start);
> +}
> +
>  static int ext3_prepare_write(struct file *file, struct page *page,
>  			      unsigned from, unsigned to)
>  {
>  	struct inode *inode = page->mapping->host;
> -	int ret, needed_blocks = ext3_writepage_trans_blocks(inode);
> +	int ret, ret2;
> +	int needed_blocks = ext3_writepage_trans_blocks(inode);
>  	handle_t *handle;
>  	int retries = 0;
>  
>  retry:
>  	handle = ext3_journal_start(inode, needed_blocks);
> -	if (IS_ERR(handle)) {
> -		ret = PTR_ERR(handle);
> -		goto out;
> -	}
> +	if (IS_ERR(handle))
> +		return PTR_ERR(handle);
>  	if (test_opt(inode->i_sb, NOBH) && ext3_should_writeback_data(inode))
>  		ret = nobh_prepare_write(page, from, to, ext3_get_block);
>  	else
>  		ret = block_prepare_write(page, from, to, ext3_get_block);
>  	if (ret)
> -		goto prepare_write_failed;
> +		goto failure;
>  
>  	if (ext3_should_journal_data(inode)) {
>  		ret = walk_page_buffers(handle, page_buffers(page),
>  				from, to, NULL, do_journal_get_write_access);
> +		if (ret)
> +			/* fatal error, just put the handle and return */
> +			journal_stop(handle);
>  	}
> -prepare_write_failed:
> -	if (ret)
> -		ext3_journal_stop(handle);
> +	return ret;
> +
> +failure:
> +	ret2 = ext3_prepare_failure(file, page, from, to);
> +	if (ret2 < 0)
> +		return ret2;
>  	if (ret == -ENOSPC && ext3_should_retry_alloc(inode->i_sb, &retries))
>  		goto retry;
> -out:
> +	/* retry number exceeded, or other error like -EDQUOT */
>  	return ret;
>  }
>  
-
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