copy: use FIEMAP (extent_copy) only for apparently-sparse files,

to avoid the expense of extent_copy's unconditional use of
FIEMAP_FLAG_SYNC.
* src/copy.c (copy_reg): Do not attempt extent_copy on a file
that appears to have no holes.
* NEWS (Changes in behavior): Document this.  At first I labeled this
as a bug fix, but that would be inaccurate, considering there is no
documentation of FIEMAP semantics, nor even consensus among kernel
FS developers.  Here's hoping SEEK_HOLE/SEEK_DATA support will soon
make it into the linux kernel.

2 files changed, 34 insertions, 16 deletions

diff --git a/NEWS b/NEWS
index 4873b5a29..7bc2ef39a 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,19 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Changes in behavior
+
+  cp's extent-based (FIEMAP) copying code is more reliable in the face
+  of varying and undocumented file system semantics:
+  - it no longer treats unwritten extents specially
+  - a FIEMAP-based extent copy always uses the FIEMAP_FLAG_SYNC flag.
+      Before, it would incur the performance penalty of that sync only
+      for 2.6.38 and older kernels.  We thought all problems would be
+      resolved for 2.6.39.
+  - it now attempts a FIEMAP copy only on a file that appears sparse.
+      Sparse files are relatively unusual, and the copying code incurs
+      the performance penalty of the now-mandatory sync only for them.
+
 
 * Noteworthy changes in release 8.11 (2011-04-13) [stable]
 
diff --git a/src/copy.c b/src/copy.c
index 3db07b5b4..6edf52efc 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -993,6 +993,7 @@ copy_reg (char const *src_name, char const *dst_name,
 
       /* Deal with sparse files.  */
       bool make_holes = false;
+      bool sparse_src = false;
 
       if (S_ISREG (sb.st_mode))
         {
@@ -1005,8 +1006,8 @@ copy_reg (char const *src_name, char const *dst_name,
              blocks.  If the file has fewer blocks than would normally be
              needed for a file of its size, then at least one of the blocks in
              the file is a hole.  */
-          if (x->sparse_mode == SPARSE_AUTO
-              && is_probably_sparse (&src_open_sb))
+          sparse_src = is_probably_sparse (&src_open_sb);
+          if (x->sparse_mode == SPARSE_AUTO && sparse_src)
             make_holes = true;
         }
 
@@ -1038,21 +1039,25 @@ copy_reg (char const *src_name, char const *dst_name,
       buf_alloc = xmalloc (buf_size + buf_alignment_slop);
       buf = ptr_align (buf_alloc, buf_alignment);
 
-      bool normal_copy_required;
-      /* Perform an efficient extent-based copy, falling back to the
-         standard copy only if the initial extent scan fails.  If the
-         '--sparse=never' option is specified, write all data but use
-         any extents to read more efficiently.  */
-      if (extent_copy (source_desc, dest_desc, buf, buf_size,
-                       src_open_sb.st_size,
-                       S_ISREG (sb.st_mode) ? x->sparse_mode : SPARSE_NEVER,
-                       src_name, dst_name, &normal_copy_required))
-        goto preserve_metadata;
-
-      if (! normal_copy_required)
+      if (sparse_src)
         {
-          return_val = false;
-          goto close_src_and_dst_desc;
+          bool normal_copy_required;
+
+          /* Perform an efficient extent-based copy, falling back to the
+             standard copy only if the initial extent scan fails.  If the
+             '--sparse=never' option is specified, write all data but use
+             any extents to read more efficiently.  */
+          if (extent_copy (source_desc, dest_desc, buf, buf_size,
+                           src_open_sb.st_size,
+                           S_ISREG (sb.st_mode) ? x->sparse_mode : SPARSE_NEVER,
+                           src_name, dst_name, &normal_copy_required))
+            goto preserve_metadata;
+
+          if (! normal_copy_required)
+            {
+              return_val = false;
+              goto close_src_and_dst_desc;
+            }
         }
 
       off_t n_read;