btrfs: Remove false alert when fiemap range is smaller than on-disk extent

Commit 4751832da990 ("btrfs: fiemap: Cache and merge fiemap extent before
submit it to user") introduced a warning to catch unemitted cached
fiemap extent.

However such warning doesn't take the following case into consideration:

0			4K			8K
|<---- fiemap range --->|
|<----------- On-disk extent ------------------>|

In this case, the whole 0~8K is cached, and since it's larger than
fiemap range, it break the fiemap extent emit loop.
This leaves the fiemap extent cached but not emitted, and caught by the
final fiemap extent sanity check, causing kernel warning.

This patch removes the kernel warning and renames the sanity check to
emit_last_fiemap_cache() since it's possible and valid to have cached
fiemap extent.

Reported-by: David Sterba <dsterba@suse.cz>
Reported-by: Adam Borowski <kilobyte@angband.pl>
Fixes: 4751832da990 ("btrfs: fiemap: Cache and merge fiemap extent ...")
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4438,29 +4438,25 @@ try_submit_last:
 }
 
 /*
- * Sanity check for fiemap cache
+ * Emit last fiemap cache
  *
- * All fiemap cache should be submitted by emit_fiemap_extent()
- * Iteration should be terminated either by last fiemap extent or
- * fieinfo->fi_extents_max.
- * So no cached fiemap should exist.
+ * The last fiemap cache may still be cached in the following case:
+ * 0		      4k		    8k
+ * |<- Fiemap range ->|
+ * |<------------  First extent ----------->|
+ *
+ * In this case, the first extent range will be cached but not emitted.
+ * So we must emit it before ending extent_fiemap().
  */
-static int check_fiemap_cache(struct btrfs_fs_info *fs_info,
-			       struct fiemap_extent_info *fieinfo,
-			       struct fiemap_cache *cache)
+static int emit_last_fiemap_cache(struct btrfs_fs_info *fs_info,
+				  struct fiemap_extent_info *fieinfo,
+				  struct fiemap_cache *cache)
 {
 	int ret;
 
 	if (!cache->cached)
 		return 0;
 
-	/* Small and recoverbale problem, only to info developer */
-#ifdef CONFIG_BTRFS_DEBUG
-	WARN_ON(1);
-#endif
-	btrfs_warn(fs_info,
-		   "unhandled fiemap cache detected: offset=%llu phys=%llu len=%llu flags=0x%x",
-		   cache->offset, cache->phys, cache->len, cache->flags);
 	ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,
 				      cache->len, cache->flags);
 	cache->cached = false;
@@ -4676,7 +4672,7 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	}
 out_free:
 	if (!ret)
-		ret = check_fiemap_cache(root->fs_info, fieinfo, &cache);
+		ret = emit_last_fiemap_cache(root->fs_info, fieinfo, &cache);
 	free_extent_map(em);
 out:
 	btrfs_free_path(path);