btrfs: integrate page status update for data read path into begin/end_page_read

In btrfs data page read path, the page status update are handled in two
different locations:

  btrfs_do_read_page()
  {
	while (cur <= end) {
		/* No need to read from disk */
		if (HOLE/PREALLOC/INLINE){
			memset();
			set_extent_uptodate();
			continue;
		}
		/* Read from disk */
		ret = submit_extent_page(end_bio_extent_readpage);
  }

  end_bio_extent_readpage()
  {
	endio_readpage_uptodate_page_status();
  }

This is fine for sectorsize == PAGE_SIZE case, as for above loop we
should only hit one branch and then exit.

But for subpage, there is more work to be done in page status update:

- Page Unlock condition
  Unlike regular page size == sectorsize case, we can no longer just
  unlock a page.
  Only the last reader of the page can unlock the page.
  This means, we can unlock the page either in the while() loop, or in
  the endio function.

- Page uptodate condition
  Since we have multiple sectors to read for a page, we can only mark
  the full page uptodate if all sectors are uptodate.

To handle both subpage and regular cases, introduce a pair of functions
to help handling page status update:

- begin_page_read()
  For regular case, it does nothing.
  For subpage case, it updates the reader counters so that later
  end_page_read() can know who is the last one to unlock the page.

- end_page_read()
  This is just endio_readpage_uptodate_page_status() renamed.
  The original name is a little too long and too specific for endio.

  The new thing added is the condition for page unlock.
  Now for subpage data, we unlock the page if we're the last reader.

This does not only provide the basis for subpage data read, but also
hide the special handling of page read from the main read loop.

Also, since we're changing how the page lock is handled, there are two
existing error paths where we need to manually unlock the page before
calling begin_page_read().

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

fs/btrfs/extent_io.c

@@ -2840,8 +2840,17 @@ update:
 	processed->uptodate = uptodate;
 }
 
-static void endio_readpage_update_page_status(struct page *page, bool uptodate,
-					      u64 start, u32 len)
+static void begin_page_read(struct btrfs_fs_info *fs_info, struct page *page)
+{
+	ASSERT(PageLocked(page));
+	if (fs_info->sectorsize == PAGE_SIZE)
+		return;
+
+	ASSERT(PagePrivate(page));
+	btrfs_subpage_start_reader(fs_info, page, page_offset(page), PAGE_SIZE);
+}
+
+static void end_page_read(struct page *page, bool uptodate, u64 start, u32 len)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(page->mapping->host->i_sb);
 
@@ -2857,7 +2866,12 @@ static void endio_readpage_update_page_status(struct page *page, bool uptodate,
 
 	if (fs_info->sectorsize == PAGE_SIZE)
 		unlock_page(page);
-	/* Subpage locking will be handled in later patches */
+	else if (is_data_inode(page->mapping->host))
+		/*
+		 * For subpage data, unlock the page if we're the last reader.
+		 * For subpage metadata, page lock is not utilized for read.
+		 */
+		btrfs_subpage_end_reader(fs_info, page, start, len);
 }
 
 /*
@@ -2994,7 +3008,7 @@ readpage_ok:
 		bio_offset += len;
 
 		/* Update page status and unlock */
-		endio_readpage_update_page_status(page, uptodate, start, len);
+		end_page_read(page, uptodate, start, len);
 		endio_readpage_release_extent(&processed, BTRFS_I(inode),
 					      start, end, uptodate);
 	}
@@ -3263,6 +3277,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 		      unsigned int read_flags, u64 *prev_em_start)
 {
 	struct inode *inode = page->mapping->host;
+	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 	u64 start = page_offset(page);
 	const u64 end = start + PAGE_SIZE - 1;
 	u64 cur = start;
@@ -3282,7 +3297,8 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 	ret = set_page_extent_mapped(page);
 	if (ret < 0) {
 		unlock_extent(tree, start, end);
-		SetPageError(page);
+		btrfs_page_set_error(fs_info, page, start, PAGE_SIZE);
+		unlock_page(page);
 		goto out;
 	}
 
@@ -3290,6 +3306,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 		if (cleancache_get_page(page) == 0) {
 			BUG_ON(blocksize != PAGE_SIZE);
 			unlock_extent(tree, start, end);
+			unlock_page(page);
 			goto out;
 		}
 	}
@@ -3306,6 +3323,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 			kunmap_atomic(userpage);
 		}
 	}
+	begin_page_read(fs_info, page);
 	while (cur <= end) {
 		bool force_bio_submit = false;
 		u64 disk_bytenr;
@@ -3323,13 +3341,14 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 					    &cached, GFP_NOFS);
 			unlock_extent_cached(tree, cur,
 					     cur + iosize - 1, &cached);
+			end_page_read(page, true, cur, iosize);
 			break;
 		}
 		em = __get_extent_map(inode, page, pg_offset, cur,
 				      end - cur + 1, em_cached);
 		if (IS_ERR_OR_NULL(em)) {
-			SetPageError(page);
 			unlock_extent(tree, cur, end);
+			end_page_read(page, false, cur, end + 1 - cur);
 			break;
 		}
 		extent_offset = cur - em->start;
@@ -3412,6 +3431,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 					    &cached, GFP_NOFS);
 			unlock_extent_cached(tree, cur,
 					     cur + iosize - 1, &cached);
+			end_page_read(page, true, cur, iosize);
 			cur = cur + iosize;
 			pg_offset += iosize;
 			continue;
@@ -3421,6 +3441,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 				   EXTENT_UPTODATE, 1, NULL)) {
 			check_page_uptodate(tree, page);
 			unlock_extent(tree, cur, cur + iosize - 1);
+			end_page_read(page, true, cur, iosize);
 			cur = cur + iosize;
 			pg_offset += iosize;
 			continue;
@@ -3429,8 +3450,8 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 		 * to date.  Error out
 		 */
 		if (block_start == EXTENT_MAP_INLINE) {
-			SetPageError(page);
 			unlock_extent(tree, cur, cur + iosize - 1);
+			end_page_read(page, false, cur, iosize);
 			cur = cur + iosize;
 			pg_offset += iosize;
 			continue;
@@ -3447,19 +3468,14 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 			nr++;
 			*bio_flags = this_bio_flag;
 		} else {
-			SetPageError(page);
 			unlock_extent(tree, cur, cur + iosize - 1);
+			end_page_read(page, false, cur, iosize);
 			goto out;
 		}
 		cur = cur + iosize;
 		pg_offset += iosize;
 	}
 out:
-	if (!nr) {
-		if (!PageError(page))
-			SetPageUptodate(page);
-		unlock_page(page);
-	}
 	return ret;
 }
 

fs/btrfs/subpage.c

@@ -54,6 +54,8 @@ int btrfs_alloc_subpage(const struct btrfs_fs_info *fs_info,
 	spin_lock_init(&(*ret)->lock);
 	if (type == BTRFS_SUBPAGE_METADATA)
 		atomic_set(&(*ret)->eb_refs, 0);
+	else
+		atomic_set(&(*ret)->readers, 0);
 	return 0;
 }
 
@@ -102,6 +104,47 @@ void btrfs_page_dec_eb_refs(const struct btrfs_fs_info *fs_info,
 	atomic_dec(&subpage->eb_refs);
 }
 
+static void btrfs_subpage_assert(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	/* Basic checks */
+	ASSERT(PagePrivate(page) && page->private);
+	ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
+	       IS_ALIGNED(len, fs_info->sectorsize));
+	/*
+	 * The range check only works for mapped page, we can still have
+	 * unmapped page like dummy extent buffer pages.
+	 */
+	if (page->mapping)
+		ASSERT(page_offset(page) <= start &&
+		       start + len <= page_offset(page) + PAGE_SIZE);
+}
+
+void btrfs_subpage_start_reader(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+	const int nbits = len >> fs_info->sectorsize_bits;
+	int ret;
+
+	btrfs_subpage_assert(fs_info, page, start, len);
+
+	ret = atomic_add_return(nbits, &subpage->readers);
+	ASSERT(ret == nbits);
+}
+
+void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+	const int nbits = len >> fs_info->sectorsize_bits;
+
+	btrfs_subpage_assert(fs_info, page, start, len);
+	ASSERT(atomic_read(&subpage->readers) >= nbits);
+	if (atomic_sub_and_test(nbits, &subpage->readers))
+		unlock_page(page);
+}
+
 /*
  * Convert the [start, start + len) range into a u16 bitmap
  *
@@ -113,18 +156,8 @@ static u16 btrfs_subpage_calc_bitmap(const struct btrfs_fs_info *fs_info,
 	const int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
 	const int nbits = len >> fs_info->sectorsize_bits;
 
-	/* Basic checks */
-	ASSERT(PagePrivate(page) && page->private);
-	ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
-	       IS_ALIGNED(len, fs_info->sectorsize));
+	btrfs_subpage_assert(fs_info, page, start, len);
 
-	/*
-	 * The range check only works for mapped page, we can still have
-	 * unmapped page like dummy extent buffer pages.
-	 */
-	if (page->mapping)
-		ASSERT(page_offset(page) <= start &&
-		       start + len <= page_offset(page) + PAGE_SIZE);
 	/*
 	 * Here nbits can be 16, thus can go beyond u16 range. We make the
 	 * first left shift to be calculate in unsigned long (at least u32),

fs/btrfs/subpage.h

@@ -29,6 +29,9 @@ struct btrfs_subpage {
 		 */
 		atomic_t eb_refs;
 		/* Structures only used by data */
+		struct {
+			atomic_t readers;
+		};
 	};
 };
 
@@ -53,6 +56,11 @@ void btrfs_page_inc_eb_refs(const struct btrfs_fs_info *fs_info,
 void btrfs_page_dec_eb_refs(const struct btrfs_fs_info *fs_info,
 			    struct page *page);
 
+void btrfs_subpage_start_reader(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len);
+void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len);
+
 /*
  * Template for subpage related operations.
  *