[PATCH] DM RAID: Ensure the bitmap region_size is a power of 2

[PATCH] DM RAID: Ensure the bitmap region_size is a power of 2

[Jonathan Brassow]

DM RAID:  Ensure the bitmap region_size is a power of 2

If the user does not supply a region_size, a reasonable size is computed
automatically.  However, region_size needs to be a power of 2 and the
code was not properly ensuring that.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>

Index: linux-upstream/drivers/md/dm-raid.c
===================================================================
--- linux-upstream.orig/drivers/md/dm-raid.c
+++ linux-upstream/drivers/md/dm-raid.c
@@ -288,6 +288,7 @@ static int dev_parms(struct raid_set *rs
  */
 static int validate_region_size(struct raid_set *rs, unsigned long region_size)
 {
+	int i;
 	unsigned long min_region_size = rs->ti->len / (1 << 21);
 
 	if (!region_size) {
@@ -295,9 +296,12 @@ static int validate_region_size(struct r
 		 * Choose a reasonable default.  All figures in sectors.
 		 */
 		if (min_region_size > (1 << 13)) {
+			/* region_size must be a power of 2 */
+			for (i = 13; min_region_size > (1 << i); i++);
+
+			region_size = (1 << i);
 			DMINFO("Choosing default region size of %lu sectors",
 			       region_size);
-			region_size = min_region_size;
 		} else {
 			DMINFO("Choosing default region size of 4MiB");
 			region_size = 1 << 13; /* sectors */

Re: [PATCH] DM RAID: Ensure the bitmap region_size is a power of 2

[Alasdair G Kergon]

On Tue, Dec 11, 2012 at 03:37:05PM -0600, Jon Brassow wrote:
>  		if (min_region_size > (1 << 13)) {
> +			/* region_size must be a power of 2 */
> +			for (i = 13; min_region_size > (1 << i); i++);
> +
> +			region_size = (1 << i);

Can the loop be replaced with a direct bit calculation, perhaps?

Alasdair

Re: [PATCH] DM RAID: Ensure the bitmap region_size is a power of 2

[Jonathan Brassow]

This is probably a better way to do it...

 brassow

DM RAID:  Ensure the bitmap region_size is a power of 2

If the user does not supply a region_size, a reasonable size is computed
automatically.  However, region_size needs to be a power of 2 and the
code was not properly ensuring that.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>

Index: linux-upstream/drivers/md/dm-raid.c
===================================================================
--- linux-upstream.orig/drivers/md/dm-raid.c
+++ linux-upstream/drivers/md/dm-raid.c
@@ -295,9 +295,11 @@ static int validate_region_size(struct r
 		 * Choose a reasonable default.  All figures in sectors.
 		 */
 		if (min_region_size > (1 << 13)) {
+			/* If not a power of 2, make it the next power of 2 */
+			if (min_region_size & (min_region_size - 1))
+				region_size = 1 << fls(region_size);
 			DMINFO("Choosing default region size of %lu sectors",
 			       region_size);
-			region_size = min_region_size;
 		} else {
 			DMINFO("Choosing default region size of 4MiB");
 			region_size = 1 << 13; /* sectors */

Re: [PATCH] DM RAID: Ensure the bitmap region_size is a power of 2

[Alasdair G Kergon]

On Wed, Dec 12, 2012 at 08:53:32AM -0600, Jon Brassow wrote:
> This is probably a better way to do it...
 
Indeed.  I'll pick this one up.

> DM RAID:  Ensure the bitmap region_size is a power of 2
> 
> If the user does not supply a region_size, a reasonable size is computed
> automatically.  However, region_size needs to be a power of 2 and the
> code was not properly ensuring that.
 
And this is just a cleanup patch: to provide a controlled error message
here, rather than triggering a controlled error inside the md code
later.

Alasdair