On 02/07/2017 10:25 PM, Shaohua Li wrote:

On Mon, Jan 30, 2017 at 07:59:48PM +0100, Artur Paszkiewicz wrote:

quoted

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.

Signed-off-by: Artur Paszkiewicz <redacted>
---
 drivers/md/raid5.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 drivers/md/raid5.h |  2 ++
 2 files changed, 100 insertions(+)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index d1cba941951e..e1e238da32ba 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c

@@ -466,6 +466,11 @@ static void shrink_buffers(struct stripe_head *sh)
 		sh->dev[i].page = NULL;
 		put_page(p);
 	}
+
+	if (sh->ppl_page) {
+		put_page(sh->ppl_page);
+		sh->ppl_page = NULL;
+	}
 }
 
 static int grow_buffers(struct stripe_head *sh, gfp_t gfp)

@@ -482,6 +487,13 @@ static int grow_buffers(struct stripe_head *sh, gfp_t gfp)
 		sh->dev[i].page = page;
 		sh->dev[i].orig_page = page;
 	}
+
+	if (test_bit(MD_HAS_PPL, &sh->raid_conf->mddev->flags)) {

I think the test should be something like:
	if (raid5_ppl_enabled())

Having the feature doesn't mean the feature is enabled.

This flag is set iff PPL is enabled, so this function would only check
the flag anyway. But I can add it to improve readability.

quoted

 	pr_debug("%s: stripe %llu locked: %d ops_request: %lx\n",
 		__func__, (unsigned long long)sh->sector,
 		s->locked, s->ops_request);

@@ -3105,6 +3175,34 @@ static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx,
 	if (*bip && (*bip)->bi_iter.bi_sector < bio_end_sector(bi))
 		goto overlap;
 
+	if (forwrite && test_bit(MD_HAS_PPL, &conf->mddev->flags)) {
+		/*
+		 * With PPL only writes to consecutive data chunks within a
+		 * stripe are allowed. Not really an overlap, but
+		 * wait_for_overlap can be used to handle this.
+		 */

Please describe why the data must be consecutive.

In PPL metadata we don't store information about which drives we write
to, only the modified data range. For a single stripe_head we can only
have one PPL entry at a time, which describes one data range. This can
be improved in the future, but will require extending the PPL metadata.

Thanks,
Artur