dm snapshot: reduce default memory allocation

Limit the amount of memory allocated per snapshot on systems
with a large page size.  (The larger default chunk size on
these systems compensates for the smaller number of pages reserved.)
Signed-off-by: NMilan Broz <mbroz@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/dm-snap.c

@@ -36,9 +36,9 @@
 #define SNAPSHOT_COPY_PRIORITY 2
 
 /*
- * Each snapshot reserves this many pages for io
+ * Reserve 1MB for each snapshot initially (with minimum of 1 page).
  */
-#define SNAPSHOT_PAGES 256
+#define SNAPSHOT_PAGES (((1UL << 20) >> PAGE_SHIFT) ? : 1)
 
 static struct workqueue_struct *ksnapd;
 static void flush_queued_bios(struct work_struct *work);