[BUG]
With expertimental subpage compression write enabled, btrfs still refuse
to compress a completely valid range:

 mkfs.btrfs -f -s 4k $dev
 mount $dev -o compress $mnt

 xfs_io -f -c "pwrite 0 64K" $mnt/file
 umount $mnt

The result extent is uncompressed.

[CAUSE]
It turns out we have extra check on whether we should compress the range
in compress_file_range().

The check looks like:

	if (nr_pages > 1 && inode_need_compress(BTRFS_I(inode), start, end)) {

This means, we will only compress range which covers more than one page.

This check no longer makes sense for subpage, as one page can contain
several sectors.

[FIX]
Don't use @nr_pages, but directly compare @total_compressed against
@blocksize to determine whether we need to do compression.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/inode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 1432e268c13e..1cab1e99e46e 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c

@@ -630,7 +630,8 @@ static noinline int compress_file_range(struct async_chunk *async_chunk)
 	 * inode has not been flagged as nocompress.  This flag can
 	 * change at any time if we discover bad compression ratios.
 	 */
-	if (nr_pages > 1 && inode_need_compress(BTRFS_I(inode), start, end)) {
+	if (total_compressed > blocksize &&
+	    inode_need_compress(BTRFS_I(inode), start, end)) {
 		WARN_ON(pages);
 		pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);
 		if (!pages) {

-- 
2.32.0