btrfs: migrate get_eb_page_index() and get_eb_offset_in_page() to folios

These two functions are still using the old page based code, which is
not going to handle larger folios at all.

The migration itself is going to involve the following changes:

- PAGE_SIZE -> folio_size()
- PAGE_SHIFT -> folio_shift()
- get_eb_page_index() -> get_eb_folio_index()
- get_eb_offset_in_page() -> get_eb_offset_in_folio()

And since we're going to support larger folios, although above straight
conversion is good enough, this patch would add extra comments in the
involved functions to explain why the same single line code can now
cover 3 cases:

- folio_size == PAGE_SIZE, sectorsize == PAGE_SIZE, nodesize >= PAGE_SIZE
  The common, non-subpage case with per-page folio.

- folio_size > PAGE_SIZE, sectorsize == PAGE_SIZE, nodesize >= PAGE_SIZE
  The incoming larger folio, non-subpage case.

- folio_size == PAGE_SIZE, sectorsize < PAGE_SIZE, nodesize < PAGE_SIZE
  The existing subpage case, we won't larger folio anyway.

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

1 files changed, 27 insertions, 13 deletions

diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index a5fd5cb20a3c..46050500529b 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -121,29 +121,43 @@ struct btrfs_eb_write_context {
  *
  * Will handle both sectorsize == PAGE_SIZE and sectorsize < PAGE_SIZE cases.
  */
-static inline size_t get_eb_offset_in_page(const struct extent_buffer *eb,
-					   unsigned long offset)
+static inline size_t get_eb_offset_in_folio(const struct extent_buffer *eb,
+					    unsigned long offset)
 {
 	/*
-	 * For sectorsize == PAGE_SIZE case, eb->start will always be aligned
-	 * to PAGE_SIZE, thus adding it won't cause any difference.
+	 * 1) sectorsize == PAGE_SIZE and nodesize >= PAGE_SIZE case
+	 *    1.1) One large folio covering the whole eb
+	 *	   The eb->start is aligned to folio size, thus adding it
+	 *	   won't cause any difference.
+	 *    1.2) Several page sized folios
+	 *	   The eb->start is aligned to folio (page) size, thus
+	 *	   adding it won't cause any difference.
 	 *
-	 * For sectorsize < PAGE_SIZE, we must only read the data that belongs
-	 * to the eb, thus we have to take the eb->start into consideration.
+	 * 2) sectorsize < PAGE_SIZE and nodesize < PAGE_SIZE case
+	 *    In this case there would only be one page sized folio, and there
+	 *    may be several different extent buffers in the page/folio.
+	 *    We need to add eb->start to properly access the offset inside
+	 *    that eb.
 	 */
-	return offset_in_page(offset + eb->start);
+	return offset_in_folio(eb->folios[0], offset + eb->start);
 }
 
-static inline unsigned long get_eb_page_index(unsigned long offset)
+static inline unsigned long get_eb_folio_index(const struct extent_buffer *eb,
+					       unsigned long offset)
 {
 	/*
-	 * For sectorsize == PAGE_SIZE case, plain >> PAGE_SHIFT is enough.
+	 * 1) sectorsize == PAGE_SIZE and nodesize >= PAGE_SIZE case
+	 *    1.1) One large folio covering the whole eb.
+	 *	   the folio_shift would be large enough to always make us
+	 *	   return 0 as index.
+	 *    1.2) Several page sized folios
+	 *         The folio_shift() would be PAGE_SHIFT, giving us the correct
+	 *         index.
 	 *
-	 * For sectorsize < PAGE_SIZE case, we only support 64K PAGE_SIZE,
-	 * and have ensured that all tree blocks are contained in one page,
-	 * thus we always get index == 0.
+	 * 2) sectorsize < PAGE_SIZE and nodesize < PAGE_SIZE case
+	 *    The folio would only be page sized, and always give us 0 as index.
 	 */
-	return offset >> PAGE_SHIFT;
+	return offset >> folio_shift(eb->folios[0]);
 }
 
 /*