[HELP] md/raid5: do we really avoid reading from known bad blocks ?

["jianchao.wang" <jianchao.w.wang@xxxxxxxxxx>]

Hi all

In analyse_stripe

		is_bad = is_badblock(rdev, sh->sector, STRIPE_SECTORS,
					     &first_bad, &bad_sectors);

		clear_bit(R5_Insync, &dev->flags);
		if (!rdev)
			/* Not in-sync */;
		else if (is_bad) {
			/* also not in-sync */
			if (!test_bit(WriteErrorSeen, &rdev->flags) &&
			    test_bit(R5_UPTODATE, &dev->flags)) {
				/* treat as in-sync, but with a read error
				 * which we can now try to correct
				 */
				set_bit(R5_Insync, &dev->flags);
				set_bit(R5_ReadError, &dev->flags);
			}
		}
If there is any bad block in the range of a read, it will fall from cache-bypass
to stripe-cache. With the R5_UPTODATE checking here, we may try to read from this
badblock instead of attempting to calculate the data directly.

Why do we need the R5_UPTODATE here ?

>From the comment of the commit, 
"
We can only treat a known-bad-block like a read-error if
we have the data that belongs in that block
"

Would anyone please help to clarify the reason ?

Thanks in advance
Jianchao