On Sat, Oct 22 2016, Song Liu wrote:

quoted

quoted

+	/* stripes only have parity are already flushed to RAID */
+	if (data_count == 0)
+		goto out;

Can you explain why that is?  When were they flushed to the RAID, and
how was the parity determined?

It happens like this: say two stripes on journal: 100 and 200. The data (D)
and parity (P) pages are store in journal as:

      --->  D100   D200   P100  P200 ---->  newer data

Before we flush D100, journal_start points as D100. Then we flush D100, 
and new journal_start points as D200. Now the system fails, so next 
recovery starts from D200. Recovery code will find stripe 100 only has
parity. This means, stripe 100 is already flushed to raid. so we can ignore it. 

OK, I see.  Thanks.

So the data for the stripe had previously been flushed before the crash
that is currently being recovered from.  It might help to make the time
of the flush more explicit in the comment:

   /* stripes that only have parity must have been flushed
    * before the crash that we are now recovering from, so
    * there is nothing more to recovery.
    */

Something like that.

Thanks,
NeilBrown