On 03/10/2017 12:24 AM, Shaohua Li wrote:

On Thu, Mar 09, 2017 at 09:59:59AM +0100, Artur Paszkiewicz wrote:

quoted

Implement the calculation of partial parity for a stripe and PPL write
logging functionality. The description of PPL is added to the
documentation. More details can be found in the comments in raid5-ppl.c.

Attach a page for holding the partial parity data to stripe_head.
Allocate it only if mddev has the MD_HAS_PPL flag set.

Partial parity is the xor of not modified data chunks of a stripe and is
calculated as follows:

- reconstruct-write case:
  xor data from all not updated disks in a stripe

- read-modify-write case:
  xor old data and parity from all updated disks in a stripe

Implement it using the async_tx API and integrate into raid_run_ops().
It must be called when we still have access to old data, so do it when
STRIPE_OP_BIODRAIN is set, but before ops_run_prexor5(). The result is
stored into sh->ppl_page.

Partial parity is not meaningful for full stripe write and is not stored
in the log or used for recovery, so don't attempt to calculate it when
stripe has STRIPE_FULL_WRITE.

Put the PPL metadata structures to md_p.h because userspace tools
(mdadm) will also need to read/write PPL.

Warn about using PPL with enabled disk volatile write-back cache for
now. It can be removed once disk cache flushing before writing PPL is
implemented.

Signed-off-by: Artur Paszkiewicz <redacted>

... snip ...

quoted

+struct dma_async_tx_descriptor *
+ops_run_partial_parity(struct stripe_head *sh, struct raid5_percpu *percpu,
+		       struct dma_async_tx_descriptor *tx)
+{
+	int disks = sh->disks;
+	struct page **xor_srcs = flex_array_get(percpu->scribble, 0);
+	int count = 0, pd_idx = sh->pd_idx, i;
+	struct async_submit_ctl submit;
+
+	pr_debug("%s: stripe %llu\n", __func__, (unsigned long long)sh->sector);
+
+	/*
+	 * Partial parity is the XOR of stripe data chunks that are not changed
+	 * during the write request. Depending on available data
+	 * (read-modify-write vs. reconstruct-write case) we calculate it
+	 * differently.
+	 */
+	if (sh->reconstruct_state == reconstruct_state_prexor_drain_run) {
+		/* rmw: xor old data and parity from updated disks */
+		for (i = disks; i--;) {
+			struct r5dev *dev = &sh->dev[i];
+			if (test_bit(R5_Wantdrain, &dev->flags) || i == pd_idx)
+				xor_srcs[count++] = dev->page;
+		}
+	} else if (sh->reconstruct_state == reconstruct_state_drain_run) {
+		/* rcw: xor data from all not updated disks */
+		for (i = disks; i--;) {
+			struct r5dev *dev = &sh->dev[i];
+			if (test_bit(R5_UPTODATE, &dev->flags))
+				xor_srcs[count++] = dev->page;
+		}
+	} else {
+		return tx;
+	}
+
+	init_async_submit(&submit, ASYNC_TX_XOR_ZERO_DST, tx, NULL, sh,
+			  flex_array_get(percpu->scribble, 0)
+			  + sizeof(struct page *) * (sh->disks + 2));

Since this should be done before biodrain, should this add ASYNC_TX_FENCE flag?

The result of this calculation isn't used later by other async_tx
operations, so it's not needed here, if I understand this correctly. But
maybe later we could optimize and use partial parity to calculate full
parity, then it will be necessary.

Thanks,
Artur