btrfs: handle page locking in btrfs_page_end_writer_lock with no writers

There are several call sites of extent_clear_unlock_delalloc() which get
@locked_page = NULL.
So that extent_clear_unlock_delalloc() will try to call
process_one_page() to unlock every page even the first page is not
locked by btrfs_page_start_writer_lock().

This will trigger an ASSERT() in btrfs_subpage_end_and_test_writer() as
previously we require every page passed to
btrfs_subpage_end_and_test_writer() to be locked by
btrfs_page_start_writer_lock().

But compression path doesn't go that way.

Thankfully it's not hard to distinguish page locked by lock_page() and
btrfs_page_start_writer_lock().

So do the check in btrfs_subpage_end_and_test_writer() so now it can
handle both cases well.

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

diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index 265b6aa..29bd8c7 100644 (file)
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c
@@ -292,6 +292,16 @@ bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_info,
 
        btrfs_subpage_assert(fs_info, page, start, len);
 
+       /*
+        * We have call sites passing @lock_page into
+        * extent_clear_unlock_delalloc() for compression path.
+        *
+        * This @locked_page is locked by plain lock_page(), thus its
+        * subpage::writers is 0.  Handle them in a special way.
+        */
+       if (atomic_read(&subpage->writers) == 0)
+               return true;
+
        ASSERT(atomic_read(&subpage->writers) >= nbits);
        return atomic_sub_and_test(nbits, &subpage->writers);
 }