btrfs: rework page locking in __extent_writepage()

Pages passed to __extent_writepage() are always locked, but they may be
locked by different functions.

There are two types of locked page for __extent_writepage():

- Page locked by plain lock_page()
  It should not have any subpage::writers count.
  Can be unlocked by unlock_page().
  This is the most common locked page for __extent_writepage() called
  inside extent_write_cache_pages() or extent_write_full_page().
  Rarer cases include the @locked_page from extent_write_locked_range().

- Page locked by lock_delalloc_pages()
  There is only one caller, all pages except @locked_page for
  extent_write_locked_range().
  In this case, we have to call subpage helper to handle the case.

So here we introduce a helper, btrfs_page_unlock_writer(), to allow
__extent_writepage() to unlock different locked pages.

And since for all other callers of __extent_writepage() their pages are
ensured to be locked by lock_page(), also add an extra check for
epd::extent_locked to unlock such pages directly.

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

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index 46224f959c34..7accb5c40d33 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -150,5 +150,7 @@ bool btrfs_subpage_clear_and_test_dirty(const struct btrfs_fs_info *fs_info,
 
 void btrfs_page_assert_not_dirty(const struct btrfs_fs_info *fs_info,
 				 struct page *page);
+void btrfs_page_unlock_writer(struct btrfs_fs_info *fs_info, struct page *page,
+			      u64 start, u32 len);
 
 #endif