ext4: add WARN_ON to check the length of allocated blocks

From: Zheng Liu <wenqing.lz@taobao.com>

In this commit (921f266b) a sanity check is added in map_blocks to make
sure 'retval == map->m_len'.  But we need to define a macro to enable
it.  This commit uses a WARN_ON to do the same thing.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
---
 fs/ext4/inode.c | 27 +++------------------------
 1 file changed, 3 insertions(+), 24 deletions(-)

On Sun, Mar 24, 2013 at 05:42:53PM +0800, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@taobao.com>
> 
> In this commit (921f266b) a sanity check is added in map_blocks to make
> sure 'retval == map->m_len'.  But we need to define a macro to enable
> it.  This commit uses a WARN_ON to do the same thing.
> 
> Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>

You and Dmitry were the ones who using originally these these checks
to fix the bugs here; if we think the code is clean enough that we
don't need the debugging information with the inode number, length,
etc., then sure, we could use the unconditionally defined WARN_ON().

If we wanted to be really paranoid and give ourselves the maximal
amount of debugging information, we could of course do something like
this:

		if (retval != map->m_len) {
			ext4_warning(inode->i_sb, "ES len assertation failed for inode: %lu retval %d != map->m_len %d\n", inode->i_ino, retval,
			       map->m_len);
			 WARN_ON(1);
		}

This way, we get the stack dump, the file system device, and all of
the debugging information.  The tradeoff is we're bloating the code
size a bit.

The question is really how confident are we that we've found all of
the potential bugs here.  If we think that there's a chance we might
trip this check in the future, sometimes it's good to print as much
information as possible, especially if it's hard to create a
reproduction on demand.

What do you think?

						- Ted
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On Sun, Mar 24, 2013 at 02:23:18PM -0400, Theodore Ts'o wrote:
> On Sun, Mar 24, 2013 at 05:42:53PM +0800, Zheng Liu wrote:
> > From: Zheng Liu <wenqing.lz@taobao.com>
> > 
> > In this commit (921f266b) a sanity check is added in map_blocks to make
> > sure 'retval == map->m_len'.  But we need to define a macro to enable
> > it.  This commit uses a WARN_ON to do the same thing.
> > 
> > Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
> 
> You and Dmitry were the ones who using originally these these checks
> to fix the bugs here;

Yes, I use this check to fix bug.

> if we think the code is clean enough that we
> don't need the debugging information with the inode number, length,
> etc., then sure, we could use the unconditionally defined WARN_ON().
> 
> If we wanted to be really paranoid and give ourselves the maximal
> amount of debugging information, we could of course do something like
> this:
> 
> 		if (retval != map->m_len) {
> 			ext4_warning(inode->i_sb, "ES len assertation failed for inode: %lu retval %d != map->m_len %d\n", inode->i_ino, retval,
> 			       map->m_len);
> 			 WARN_ON(1);
> 		}

I think this is better.

> 
> This way, we get the stack dump, the file system device, and all of
> the debugging information.  The tradeoff is we're bloating the code
> size a bit.
> 
> The question is really how confident are we that we've found all of
> the potential bugs here.  If we think that there's a chance we might
> trip this check in the future, sometimes it's good to print as much
> information as possible, especially if it's hard to create a
> reproduction on demand.
> 
> What do you think?

In my sand box, after fixed the bug, I never see this warning again.
But I do believe we'd better leave it here to give us an opportunity to
fix some potential bugs.

Thanks,
                                                - Zheng
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