btrfs: subpage: pack all subpage bitmaps into a larger bitmap

Currently we use u16 bitmap to make 4k sectorsize work for 64K page
size.

But this u16 bitmap is not large enough to contain larger page size like
128K, nor is space efficient for 16K page size.

To handle both cases, here we pack all subpage bitmaps into a larger
bitmap, now btrfs_subpage::bitmaps[] will be the ultimate bitmap for
subpage usage.

Each sub-bitmap will has its start bit number recorded in
btrfs_subpage_info::*_start, and its bitmap length will be recorded in
btrfs_subpage_info::bitmap_nr_bits.

All subpage bitmap operations will be converted from using direct u16
operations to bitmap operations, with above *_start calculated.

For 64K page size with 4K sectorsize, this should not cause much
difference.

While for 16K page size, we will only need 1 unsigned long (u32) to
store all the bitmaps, which saves quite some space.

Furthermore, this allows us to support larger page size like 128K and
258K.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 3 insertions, 16 deletions

diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index 386246a7770f..ac4dd64ed257 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -6,12 +6,6 @@
 #include <linux/spinlock.h>
 
 /*
- * Maximum page size we support is 64K, minimum sector size is 4K, u16 bitmap
- * is sufficient. Regular bitmap_* is not used due to size reasons.
- */
-#define BTRFS_SUBPAGE_BITMAP_SIZE	16
-
-/*
  * Extra info for subpapge bitmap.
  *
  * For subpage we pack all uptodate/error/dirty/writeback/ordered bitmaps into
@@ -51,10 +45,6 @@ struct btrfs_subpage_info {
 struct btrfs_subpage {
 	/* Common members for both data and metadata pages */
 	spinlock_t lock;
-	u16 uptodate_bitmap;
-	u16 error_bitmap;
-	u16 dirty_bitmap;
-	u16 writeback_bitmap;
 	/*
 	 * Both data and metadata needs to track how many readers are for the
 	 * page.
@@ -71,14 +61,11 @@ struct btrfs_subpage {
 		 * manages whether the subpage can be detached.
 		 */
 		atomic_t eb_refs;
-		/* Structures only used by data */
-		struct {
-			atomic_t writers;
 
-			/* Tracke pending ordered extent in this sector */
-			u16 ordered_bitmap;
-		};
+		/* Structures only used by data */
+		atomic_t writers;
 	};
+	unsigned long bitmaps[];
 };
 
 enum btrfs_subpage_type {