ext4: reimplement uninit extent optimization for move_extent_per_page()

Uninitialized extent may became initialized(parallel writeback task)
at any moment after we drop i_data_sem, so we have to recheck extent's
state after we hold page's lock and i_data_sem.

If we about to change page's mapping we must hold page's lock in order to
serialize other users.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 76 insertions, 5 deletions

diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index c87a746450e5..c2e47da7c2ba 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c
@@ -595,6 +595,43 @@ mext_calc_swap_extents(struct ext4_extent *tmp_dext,
 }
 
 /**
+ * mext_check_coverage - Check that all extents in range has the same type
+ *
+ * @inode:              inode in question
+ * @from:               block offset of inode
+ * @count:              block count to be checked
+ * @uninit:             extents expected to be uninitialized
+ * @err:                pointer to save error value
+ *
+ * Return 1 if all extents in range has expected type, and zero otherwise.
+ */
+static int
+mext_check_coverage(struct inode *inode, ext4_lblk_t from, ext4_lblk_t count,
+                          int uninit, int *err)
+{
+        struct ext4_ext_path *path = NULL;
+        struct ext4_extent *ext;
+        ext4_lblk_t last = from + count;
+        while (from < last) {
+                *err = get_ext_path(inode, from, &path);
+                if (*err)
+                        return 0;
+                ext = path[ext_depth(inode)].p_ext;
+                if (!ext) {
+                        ext4_ext_drop_refs(path);
+                        return 0;
+                }
+                if (uninit != ext4_ext_is_uninitialized(ext)) {
+                        ext4_ext_drop_refs(path);
+                        return 0;
+                }
+                from += ext4_ext_get_actual_len(ext);
+                ext4_ext_drop_refs(path);
+        }
+        return 1;
+}
+
+/**
  * mext_replace_branches - Replace original extents with new extents
  *
  * @handle:             journal handle
@@ -629,9 +666,6 @@ mext_replace_branches(handle_t *handle, struct inode *orig_inode,
         int replaced_count = 0;
         int dext_alen;
 
-        /* Protect extent trees against block allocations via delalloc */
-        double_down_write_data_sem(orig_inode, donor_inode);
-
         /* Get the original extent for the block "orig_off" */
         *err = get_ext_path(orig_inode, orig_off, &orig_path);
         if (*err)
@@ -730,8 +764,6 @@ out:
         ext4_ext_invalidate_cache(orig_inode);
         ext4_ext_invalidate_cache(donor_inode);
 
-        double_up_write_data_sem(orig_inode, donor_inode);
-
         return replaced_count;
 }
 
@@ -925,7 +957,46 @@ again:
                                      pagep);
         if (unlikely(*err < 0))
                 goto stop_journal;
+        /*
+         * If orig extent was uninitialized it can become initialized
+         * at any time after i_data_sem was dropped, in order to
+         * serialize with delalloc we have recheck extent while we
+         * hold page's lock, if it is still the case data copy is not
+         * necessary, just swap data blocks between orig and donor.
+         */
+        if (uninit) {
+                double_down_write_data_sem(orig_inode, donor_inode);
+                /* If any of extents in range became initialized we have to
+                 * fallback to data copying */
+                uninit = mext_check_coverage(orig_inode, orig_blk_offset,
+                                             block_len_in_page, 1, err);
+                if (*err)
+                        goto drop_data_sem;
 
+                uninit &= mext_check_coverage(donor_inode, orig_blk_offset,
+                                              block_len_in_page, 1, err);
+                if (*err)
+                        goto drop_data_sem;
+
+                if (!uninit) {
+                        double_up_write_data_sem(orig_inode, donor_inode);
+                        goto data_copy;
+                }
+                if ((page_has_private(pagep[0]) &&
+                     !try_to_release_page(pagep[0], 0)) ||
+                    (page_has_private(pagep[1]) &&
+                     !try_to_release_page(pagep[1], 0))) {
+                        *err = -EBUSY;
+                        goto drop_data_sem;
+                }
+                replaced_count = mext_replace_branches(handle, orig_inode,
+                                                donor_inode, orig_blk_offset,
+                                                block_len_in_page, err);
+        drop_data_sem:
+                double_up_write_data_sem(orig_inode, donor_inode);
+                goto unlock_pages;
+        }
+data_copy:
         *err = mext_page_mkuptodate(pagep[0], from, from + replaced_size);
         if (*err)
                 goto unlock_pages;