Re: [PATCH] retries in ext3_prepare_write() violate ordering requirements

[Kirill Korotaev <dev@openvz.org>]

Andrew,

Dmitry confirmed that it works in NOBH mode as well
and when quota limit is hit.

Thanks,
Kirill

> Andrew,
> 
> 
>>>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?)
> 
> purely theoretically while hunting for other bugs related to ext3 and quota.
> block_prepare_write() can fail e.g. if quota returns -EDQUOT in ext3_alloc_blocks().
> 
> 
>>How was the patch tested?
> 
> 1. it was tested as part of OpenVZ kernel
> 2. there were ext3 stress test done with lots of disk activity by Dmitry Monakhov.
> 
> 
>>Was nobh-mode also tested?
> 
> I will ask to perform some more tests 100% triggering ext3_prepare_failure()
> and with NOBH mode.
> 
> Thanks,
> Kirill
> 
> 
> 
>>>--- ./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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