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
restore the bitmap.
And will be able to support 128K page size in the future.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/extent_io.c |  58 ++++++++++--------
 fs/btrfs/subpage.c   | 137 +++++++++++++++++++++++++++++--------------
 fs/btrfs/subpage.h   |  19 +-----
 3 files changed, 130 insertions(+), 84 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 543f87ea372e..e428d6208bb7 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c

@@ -3865,10 +3865,9 @@ static void find_next_dirty_byte(struct btrfs_fs_info *fs_info,
 	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
 	u64 orig_start = *start;
 	/* Declare as unsigned long so we can use bitmap ops */
-	unsigned long dirty_bitmap;
 	unsigned long flags;
-	int nbits = (orig_start - page_offset(page)) >> fs_info->sectorsize_bits;
-	int range_start_bit = nbits;
+	int nbits = offset_in_page(orig_start) >> fs_info->sectorsize_bits;

+	int range_start_bit = nbits + fs_info->subpage_info->dirty_start;
 	int range_end_bit;
 
 	/*

@@ -3883,11 +3882,14 @@ static void find_next_dirty_byte(struct btrfs_fs_info *fs_info,
 
 	/* We should have the page locked, but just in case */
 	spin_lock_irqsave(&subpage->lock, flags);
-	dirty_bitmap = subpage->dirty_bitmap;
+	bitmap_next_set_region(subpage->bitmaps, &range_start_bit, &range_end_bit,
+			       fs_info->subpage_info->dirty_start +
+			       fs_info->subpage_info->bitmap_nr_bits);
 	spin_unlock_irqrestore(&subpage->lock, flags);
 
-	bitmap_next_set_region(&dirty_bitmap, &range_start_bit, &range_end_bit,
-			       BTRFS_SUBPAGE_BITMAP_SIZE);
+	range_start_bit -= fs_info->subpage_info->dirty_start;
+	range_end_bit -= fs_info->subpage_info->dirty_start;
+
 	*start = page_offset(page) + range_start_bit * fs_info->sectorsize;
 	*end = page_offset(page) + range_end_bit * fs_info->sectorsize;
 }

@@ -4613,7 +4615,7 @@ static int submit_eb_subpage(struct page *page,
 	int submitted = 0;
 	u64 page_start = page_offset(page);
 	int bit_start = 0;
-	const int nbits = BTRFS_SUBPAGE_BITMAP_SIZE;
+	const int nbits = fs_info->subpage_info->bitmap_nr_bits;

@@ -7178,32 +7181,41 @@ void memmove_extent_buffer(const struct extent_buffer *dst,
 	}
 }
 
+#define GANG_LOOKUP_SIZE	16
 static struct extent_buffer *get_next_extent_buffer(
 		struct btrfs_fs_info *fs_info, struct page *page, u64 bytenr)
 {
-	struct extent_buffer *gang[BTRFS_SUBPAGE_BITMAP_SIZE];
+	struct extent_buffer *gang[GANG_LOOKUP_SIZE];
 	struct extent_buffer *found = NULL;
 	u64 page_start = page_offset(page);
-	int ret;
-	int i;
+	u64 cur = page_start;
 
 	ASSERT(in_range(bytenr, page_start, PAGE_SIZE));
-	ASSERT(PAGE_SIZE / fs_info->nodesize <= BTRFS_SUBPAGE_BITMAP_SIZE);
 	lockdep_assert_held(&fs_info->buffer_lock);
 
-	ret = radix_tree_gang_lookup(&fs_info->buffer_radix, (void **)gang,
-			bytenr >> fs_info->sectorsize_bits,
-			PAGE_SIZE / fs_info->nodesize);
-	for (i = 0; i < ret; i++) {
-		/* Already beyond page end */
-		if (gang[i]->start >= page_start + PAGE_SIZE)
-			break;
-		/* Found one */
-		if (gang[i]->start >= bytenr) {
-			found = gang[i];
-			break;
+	while (cur < page_start + PAGE_SIZE) {
+		int ret;
+		int i;
+
+		ret = radix_tree_gang_lookup(&fs_info->buffer_radix,
+				(void **)gang, cur >> fs_info->sectorsize_bits,
+				min_t(unsigned int, GANG_LOOKUP_SIZE,
+				      PAGE_SIZE / fs_info->nodesize));
+		if (ret == 0)
+			goto out;
+		for (i = 0; i < ret; i++) {
+			/* Already beyond page end */
+			if (gang[i]->start >= page_start + PAGE_SIZE)
+				goto out;
+			/* Found one */
+			if (gang[i]->start >= bytenr) {
+				found = gang[i];
+				break;

+			}
 		}
+		cur = gang[ret - 1]->start + gang[ret - 1]->len;
 	}
+out:
 	return found;
 }
 

diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index 014256d47beb..9d6da9f2d77e 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c

@@ -139,10 +139,14 @@ int btrfs_alloc_subpage(const struct btrfs_fs_info *fs_info,
 			struct btrfs_subpage **ret,
 			enum btrfs_subpage_type type)
 {
+	unsigned int real_size;
+
 	if (fs_info->sectorsize == PAGE_SIZE)
 		return 0;

+	*ret = kzalloc(real_size, GFP_NOFS);
 	if (!*ret)
 		return -ENOMEM;
 	spin_lock_init(&(*ret)->lock);

@@ -324,37 +328,60 @@ void btrfs_page_end_writer_lock(const struct btrfs_fs_info *fs_info,
 		unlock_page(page);
 }
 
-/*
- * Convert the [start, start + len) range into a u16 bitmap
- *
- * For example: if start == page_offset() + 16K, len = 16K, we get 0x00f0.
- */
-static u16 btrfs_subpage_calc_bitmap(const struct btrfs_fs_info *fs_info,
-		struct page *page, u64 start, u32 len)
+static bool bitmap_test_range_all_set(unsigned long *addr, unsigned start,
+				      unsigned int nbits)
 {
-	const int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
-	const int nbits = len >> fs_info->sectorsize_bits;
+	unsigned int found_zero;
 
-	btrfs_subpage_assert(fs_info, page, start, len);
+	found_zero = find_next_zero_bit(addr, start + nbits, start);