linux-kernel - Re: [PATCH] fix hard-coded disk header chunk size for persistent snapshot

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Date:	Wed, 12 May 2010 14:57:34 +0900
From:	Tomohiro Kusumi <kusumi.tomohiro@...fujitsu.com>
To:	Mikulas Patocka <mpatocka@...hat.com>,
	Alasdair G Kergon <agk@...hat.com>, dm-devel@...hat.com
CC:	linux-kernel@...r.kernel.org, kusumi.tomohiro@...fujitsu.com
Subject: Re: [PATCH] fix hard-coded disk header chunk size for persistent
 snapshot

(2010/05/12 13:58), Mikulas Patocka wrote:
>
>
> On Wed, 12 May 2010, Alasdair G Kergon wrote:
>
>> On Wed, May 12, 2010 at 10:29:22AM +0900, Tomohiro Kusumi wrote:
>>> This patch fixes hard-coded value for the size of a chunk that includes
>>> disk header for persistent snapshot. It should be changed to existing
>>> macro NUM_SNAPSHOT_HDR_CHUNKS instead of using hard-coded value 1.
>>
>> Any more places too?  (E.g. persistent_commit_merge)
>>
>> Alasdair
>
> I think the expression in persistent_commit_merge should be changed to:
> ps->next_free = area_location(ps, ps->current_area) +
>                  ps->current_committed + 1;
>
> It is numerically the same, but will avoid trouble if
> NUM_SNAPSHOT_HDR_CHUNKS is increased.
>
> Mikulas
>
>

agree,

a single chunk is large enough to contain 16byte disk header
which does not seem necessary to increase NUM_SNAPSHOT_HDR_CHUNKS in the future
(unless whole structure get totally changed),
but it should also get fixed to avoid trouble.

I merged it to the patch with your names signed off.

Thanks,
Tomohiro Kusumi


Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@...fujitsu.com>
Signed-off-by: Alasdair G Kergon <agk@...hat.com>
Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
---

diff -aNur linux-2.6.34-rc7.org/drivers/md/dm-snap-persistent.c linux-2.6.34-rc7/drivers/md/dm-snap-persistent.c
--- linux-2.6.34-rc7.org/drivers/md/dm-snap-persistent.c	2010-05-10 10:36:28.000000000 +0900
+++ linux-2.6.34-rc7/drivers/md/dm-snap-persistent.c	2010-05-12 14:49:13.000000000 +0900
@@ -266,7 +266,7 @@
   */
  static chunk_t area_location(struct pstore *ps, chunk_t area)
  {
-	return 1 + ((ps->exceptions_per_area + 1) * area);
+	return NUM_SNAPSHOT_HDR_CHUNKS + ((ps->exceptions_per_area + 1) * area);
  }
  
  /*
@@ -780,8 +780,8 @@
  	 * ps->current_area does not get reduced by prepare_merge() until
  	 * after commit_merge() has removed the nr_merged previous exceptions.
  	 */
-	ps->next_free = (area_location(ps, ps->current_area) - 1) +
-			(ps->current_committed + 1) + NUM_SNAPSHOT_HDR_CHUNKS;
+	ps->next_free = area_location(ps, ps->current_area) +
+			ps->current_committed + 1;
  
  	return 0;
  }

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