fiemap: single-ioctl rescan in fiemap_scan_extent

fiemap.c

@@ -152,6 +152,38 @@ struct fiemap *do_fiemap_range(int fd, uint64_t start, uint64_t length)
 	return fiemap;
 }
 
+/*
+ * Physical offset of the first extent overlapping [start, start+length).
+ *
+ * The dedupe rescan only needs that one extent, so unlike do_fiemap_range()
+ * this issues a single ioctl (no separate count pass) into a one-extent buffer.
+ * Returns 0 and stores the offset in *poff on success, -1 on ioctl error or
+ * when the range maps no extents (a hole).
+ */
+int fiemap_first_extent_poff(int fd, uint64_t start, uint64_t length,
+			     uint64_t *poff)
+{
+	struct {
+		struct fiemap		fiemap;
+		struct fiemap_extent	extent;
+	} buf = {0,};
+
+	buf.fiemap.fm_start = start;
+	buf.fiemap.fm_length = length;
+	buf.fiemap.fm_extent_count = 1;
+
+	if (ioctl(fd, FS_IOC_FIEMAP, &buf.fiemap) < 0) {
+		perror("fiemap_first_extent_poff");
+		return -1;
+	}
+
+	if (buf.fiemap.fm_mapped_extents == 0)
+		return -1;
+
+	*poff = buf.fiemap.fm_extents[0].fe_physical;
+	return 0;
+}
+
 int fiemap_count_shared(int fd, size_t start_off, size_t end_off, uint64_t *shared)
 {
 	_cleanup_(freep) struct fiemap *fiemap = NULL;

fiemap.h

@@ -31,6 +31,14 @@ struct fiemap *do_fiemap(int fd);
  */
 struct fiemap *do_fiemap_range(int fd, uint64_t start, uint64_t length);
 
+/*
+ * Physical offset of the first extent overlapping [start, start+length), via a
+ * single fiemap ioctl. Returns 0 and sets *poff on success, -1 on error or when
+ * the range maps no extents.
+ */
+int fiemap_first_extent_poff(int fd, uint64_t start, uint64_t length,
+			     uint64_t *poff);
+
 /*
  * Count how much of the area between start_off and end_off is shared.
  */

filerec.c

@@ -315,24 +315,17 @@ void filerec_close_open_list(struct open_once *open_files)
 int fiemap_scan_extent(struct extent *extent)
 {
 	int ret = 0;
-	_cleanup_(freep) struct fiemap *fiemap = NULL;
 
 	ret = filerec_open(extent->e_file, true);
 	if (ret)
 		return ret;
 
 	/*
-	 * Only map the extent we care about. On a large/fragmented file,
-	 * do_fiemap() would enumerate every extent just to fetch this one.
+	 * We only need this one extent's physical offset. On a large/fragmented
+	 * file, do_fiemap() would enumerate every extent just to fetch it.
 	 */
-	fiemap = do_fiemap_range(extent->e_file->fd, extent->e_loff,
-				 extent_len(extent));
-	if (!fiemap || fiemap->fm_mapped_extents == 0) {
-		filerec_close(extent->e_file);
-		return -1;
-	}
-
-	extent->e_poff = fiemap->fm_extents[0].fe_physical;
+	ret = fiemap_first_extent_poff(extent->e_file->fd, extent->e_loff,
+				       extent_len(extent), &extent->e_poff);
 	filerec_close(extent->e_file);
 	return ret;
 }