Re: [PATCH 5/5] writeback: check skipped pages on WB_SYNC_ALL

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 09 Nov 2010 07:09:21 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> In WB_SYNC_ALL mode, filesystems are not expected to skip dirty pages on
> temporal lock contentions or non fatal errors, otherwise sync() will
> return without actually syncing the skipped pages. Add a check to
> catch possible redirty_page_for_writepage() callers that violate this
> expectation.
> 
> I'd recommend to keep this check in -mm tree for some time and fixup the
> possible warnings before pushing it to upstream.
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  fs/fs-writeback.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> --- linux-next.orig/fs/fs-writeback.c	2010-11-07 22:01:06.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-11-07 22:01:15.000000000 +0800
> @@ -527,6 +527,7 @@ static int writeback_sb_inodes(struct su
>  			 * buffers.  Skip this inode for now.
>  			 */
>  			redirty_tail(inode);
> +			WARN_ON_ONCE(wbc->sync_mode == WB_SYNC_ALL);
>  		}
>  		spin_unlock(&inode_lock);
>  		iput(inode);

This is quite kernel-developer-unfriendly.

Suppose the warning triggers.  Now some poor schmuck looks at the
warning and doesn't have a *clue* why it was added.  He has to run off
and grovel through git trees finding changelogs, which is a real pain
if the code has been trivially altered since it was first added.

As a general rule, a kernel developer should be able to look at a
warning callsite and then work out why the warning was emitted!


IOW, you owe us a code comment, please.

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