btrfs: only unlock the to-be-submitted ranges inside a folio

[SUBPAGE COMPRESSION LIMITS]
Currently inside writepage_delalloc(), if a delalloc range is going to
be submitted asynchronously (inline or compression, the page
dirty/writeback/unlock are all handled in at different time, not at the
submission time), then we return 1 and extent_writepage() will skip the
submission.

This is fine if every sector matches page size, but if a sector is
smaller than page size (aka, subpage case), then it can be very
problematic, for example for the following 64K page:

     0     16K     32K    48K     64K
     |/|   |///////|      |/|
       |                    |
       4K                   52K

Where |/| is the dirty range we need to submit.

In the above case, we need the following different handling for the 3
ranges:

- [0, 4K) needs to be submitted for regular write
  A single sector cannot be compressed.

- [16K, 32K) needs to be submitted for compressed write

- [48K, 52K) needs to be submitted for regular write.

Above, if we try to submit [16K, 32K) for compressed write, we will
return 1 and immediately, and without submitting the remaining
[48K, 52K) range.

Furthermore, since extent_writepage() will exit without unlocking any
sectors, the submitted range [0, 4K) will not have sector unlocked.

That's the reason why for now subpage is only allowed for full page
range.

[ENHANCEMENT]
- Introduce a submission bitmap at btrfs_bio_ctrl::submit_bitmap
  This records which sectors will be submitted by extent_writepage_io().
  This allows us to track which sectors needs to be submitted thus later
  to be properly unlocked.

  For asynchronously submitted range (inline/compression), the
  corresponding bits will be cleared from that bitmap.

- Only return 1 if no sector needs to be submitted in
  writepage_delalloc()

- Only submit sectors marked by submission bitmap inside
  extent_writepage_io()
  So we won't touch the asynchronously submitted part.

- Introduce btrfs_folio_end_writer_lock_bitmap() helper
  This will only unlock the involved sectors specified by @bitmap
  parameter, to avoid touching the range asynchronously submitted.

Please note that, since subpage compression is still limited to page
aligned range, this change is only a preparation for future sector
perfect compression support for subpage.

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

1 files changed, 33 insertions, 0 deletions

diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index 83660fa82c32..fe4d719d506b 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c
@@ -424,6 +424,39 @@ void btrfs_folio_end_writer_lock(const struct btrfs_fs_info *fs_info,
 		folio_unlock(folio);
 }
 
+void btrfs_folio_end_writer_lock_bitmap(const struct btrfs_fs_info *fs_info,
+					struct folio *folio, unsigned long bitmap)
+{
+	struct btrfs_subpage *subpage = folio_get_private(folio);
+	const int start_bit = fs_info->sectors_per_page * btrfs_bitmap_nr_locked;
+	unsigned long flags;
+	bool last = false;
+	int cleared = 0;
+	int bit;
+
+	if (unlikely(!fs_info) || !btrfs_is_subpage(fs_info, folio->mapping)) {
+		folio_unlock(folio);
+		return;
+	}
+
+	if (atomic_read(&subpage->writers) == 0) {
+		/* No writers, locked by plain lock_page(). */
+		folio_unlock(folio);
+		return;
+	}
+
+	spin_lock_irqsave(&subpage->lock, flags);
+	for_each_set_bit(bit, &bitmap, fs_info->sectors_per_page) {
+		if (test_and_clear_bit(bit + start_bit, subpage->bitmaps))
+			cleared++;
+	}
+	ASSERT(atomic_read(&subpage->writers) >= cleared);
+	last = atomic_sub_and_test(cleared, &subpage->writers);
+	spin_unlock_irqrestore(&subpage->lock, flags);
+	if (last)
+		folio_unlock(folio);
+}
+
 #define subpage_test_bitmap_all_set(fs_info, subpage, name)		\
 	bitmap_test_range_all_set(subpage->bitmaps,			\
 			fs_info->sectors_per_page * btrfs_bitmap_nr_##name, \