btrfs: retry verity reads for not-uptodate Merkle folios

btrfs_read_merkle_tree_page() can find a folio in the mapping that is not
uptodate.  After taking the folio lock, the current code treats that state
as a read error and returns -EIO.

That can make a previous transient read failure sticky.  If the failed read
left a not-uptodate folio in the mapping, later callers find that folio and
fail instead of retrying the read.

Keep the existing page-cache insertion and locking order, but retry the
Merkle item read when a not-uptodate folio is found in the mapping.  Also
unlock the folio when read_key_bytes() fails so that a later caller can
lock it and retry the read.

Fixes: 06ed09351b ("btrfs: convert btrfs_read_merkle_tree_page() to use a folio")
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Yichong Chen <chenyichong@uniontech.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/verity.c

@@ -720,14 +720,18 @@ again:
 			goto out;
 
 		folio_lock(folio);
-		/* If it's not uptodate after we have the lock, we got a read error. */
-		if (!folio_test_uptodate(folio)) {
+		/* Folio was truncated from mapping. */
+		if (!folio->mapping) {
 			folio_unlock(folio);
 			folio_put(folio);
-			return ERR_PTR(-EIO);
+			goto again;
 		}
-		folio_unlock(folio);
-		goto out;
+		/* Another reader may have filled the folio while we waited. */
+		if (folio_test_uptodate(folio)) {
+			folio_unlock(folio);
+			goto out;
+		}
+		goto read_folio;
 	}
 
 	folio = filemap_alloc_folio(mapping_gfp_constraint(inode->i_mapping, ~__GFP_FS),
@@ -744,6 +748,7 @@ again:
 		return ERR_PTR(ret);
 	}
 
+read_folio:
 	/*
 	 * Merkle item keys are indexed from byte 0 in the merkle tree.
 	 * They have the form:
@@ -753,6 +758,7 @@ again:
 	ret = read_key_bytes(BTRFS_I(inode), BTRFS_VERITY_MERKLE_ITEM_KEY, off,
 			     folio_address(folio), PAGE_SIZE, folio);
 	if (ret < 0) {
+		folio_unlock(folio);
 		folio_put(folio);
 		return ERR_PTR(ret);
 	}