md: Allow write-intent bitmaps to have chunksize < PAGE_SIZE

md currently insists that the chunk size used for write-intent
bitmaps (the amount of data that corresponds to one chunk)
be at least one page.

The reason for this restriction is lost in the mists of time,
but a review of the code (and a vague memory) suggests that the only
problem would be related to resync.  Resync tries very hard to
work in multiples of a page, but also needs to sync with units
of a bitmap_chunk too.

This connection comes out in the bitmap_start_sync call.

So change bitmap_start_sync to always work in multiples of a page.
If the bitmap chunk size is less that one page, we flag multiple
chunks as 'syncing' and generally make them all appear to the
resync routines like one chunk.

All other code either already works with data ranges that could
span multiple chunks, or explicitly only cares about a single chunk.
Signed-off-by: Neil Brown <neilb@suse.de>

drivers/md/bitmap.c

@@ -111,9 +111,10 @@ static int bitmap_checkpage(struct bitmap *bitmap, unsigned long page, int creat
 	unsigned char *mappage;
 
 	if (page >= bitmap->pages) {
-		printk(KERN_ALERT
-			"%s: invalid bitmap page request: %lu (> %lu)\n",
-			bmname(bitmap), page, bitmap->pages-1);
+		/* This can happen if bitmap_start_sync goes beyond
+		 * End-of-device while looking for a whole page.
+		 * It is harmless.
+		 */
 		return -EINVAL;
 	}
 
@@ -570,7 +571,7 @@ static int bitmap_read_sb(struct bitmap *bitmap)
 	else if (le32_to_cpu(sb->version) < BITMAP_MAJOR_LO ||
 		 le32_to_cpu(sb->version) > BITMAP_MAJOR_HI)
 		reason = "unrecognized superblock version";
-	else if (chunksize < PAGE_SIZE)
+	else if (chunksize < 512)
 		reason = "bitmap chunksize too small";
 	else if ((1 << ffz(~chunksize)) != chunksize)
 		reason = "bitmap chunksize not a power of 2";
@@ -1345,8 +1346,8 @@ void bitmap_endwrite(struct bitmap *bitmap, sector_t offset, unsigned long secto
 	}
 }
 
-int bitmap_start_sync(struct bitmap *bitmap, sector_t offset, int *blocks,
-			int degraded)
+static int __bitmap_start_sync(struct bitmap *bitmap, sector_t offset, int *blocks,
+			       int degraded)
 {
 	bitmap_counter_t *bmc;
 	int rv;
@@ -1374,6 +1375,29 @@ int bitmap_start_sync(struct bitmap *bitmap, sector_t offset, int *blocks,
 	return rv;
 }
 
+int bitmap_start_sync(struct bitmap *bitmap, sector_t offset, int *blocks,
+		      int degraded)
+{
+	/* bitmap_start_sync must always report on multiples of whole
+	 * pages, otherwise resync (which is very PAGE_SIZE based) will
+	 * get confused.
+	 * So call __bitmap_start_sync repeatedly (if needed) until
+	 * At least PAGE_SIZE>>9 blocks are covered.
+	 * Return the 'or' of the result.
+	 */
+	int rv = 0;
+	int blocks1;
+
+	*blocks = 0;
+	while (*blocks < (PAGE_SIZE>>9)) {
+		rv |= __bitmap_start_sync(bitmap, offset,
+					  &blocks1, degraded);
+		offset += blocks1;
+		*blocks += blocks1;
+	}
+	return rv;
+}
+
 void bitmap_end_sync(struct bitmap *bitmap, sector_t offset, int *blocks, int aborted)
 {
 	bitmap_counter_t *bmc;