btrfs: improve comments around nocow path

run_delalloc_nocow contains numerous, somewhat subtle, checks when
figuring out whether a particular extent should be CoW'ed or not. This
patch explicitly states the assumptions those checks verify. As a
result also document 2 of the more subtle checks in check_committed_ref
as well.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5e8c6a0..af76314 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2357,16 +2357,19 @@ static noinline int check_committed_ref(struct btrfs_root *root,
        item_size = btrfs_item_size_nr(leaf, path->slots[0]);
        ei = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_extent_item);
 
+       /* If extent item has more than 1 inline ref then it's shared */
        if (item_size != sizeof(*ei) +
            btrfs_extent_inline_ref_size(BTRFS_EXTENT_DATA_REF_KEY))
                goto out;
 
+       /* If extent created before last snapshot => it's definitely shared */
        if (btrfs_extent_generation(leaf, ei) <=
            btrfs_root_last_snapshot(&root->root_item))
                goto out;
 
        iref = (struct btrfs_extent_inline_ref *)(ei + 1);
 
+       /* If this extent has SHARED_DATA_REF then it's shared */
        type = btrfs_get_extent_inline_ref_type(leaf, iref, BTRFS_REF_TYPE_DATA);
        if (type != BTRFS_EXTENT_DATA_REF_KEY)
                goto out;

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index fdc20d8..ae44ec2 100644 (file)
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1345,6 +1345,12 @@ static noinline int run_delalloc_nocow(struct inode *inode,
                                               cur_offset, 0);
                if (ret < 0)
                        goto error;
+
+               /*
+                * If there is no extent for our range when doing the initial
+                * search, then go back to the previous slot as it will be the
+                * one containing the search offset
+                */
                if (ret > 0 && path->slots[0] > 0 && check_prev) {
                        leaf = path->nodes[0];
                        btrfs_item_key_to_cpu(leaf, &found_key,
@@ -1355,6 +1361,7 @@ static noinline int run_delalloc_nocow(struct inode *inode,
                }
                check_prev = false;
 next_slot:
+               /* Go to next leaf if we have exhausted the current one */
                leaf = path->nodes[0];
                if (path->slots[0] >= btrfs_header_nritems(leaf)) {
                        ret = btrfs_next_leaf(root, path);
@@ -1370,23 +1377,38 @@ next_slot:
 
                btrfs_item_key_to_cpu(leaf, &found_key, path->slots[0]);
 
+               /* Didn't find anything for our INO */
                if (found_key.objectid > ino)
                        break;
+               /*
+                * Keep searching until we find an EXTENT_ITEM or there are no
+                * more extents for this inode
+                */
                if (WARN_ON_ONCE(found_key.objectid < ino) ||
                    found_key.type < BTRFS_EXTENT_DATA_KEY) {
                        path->slots[0]++;
                        goto next_slot;
                }
+
+               /* Found key is not EXTENT_DATA_KEY or starts after req range */
                if (found_key.type > BTRFS_EXTENT_DATA_KEY ||
                    found_key.offset > end)
                        break;
 
+               /*
+                * If the found extent starts after requested offset, then
+                * adjust extent_end to be right before this extent begins
+                */
                if (found_key.offset > cur_offset) {
                        extent_end = found_key.offset;
                        extent_type = 0;
                        goto out_check;
                }
 
+               /*
+                * Found extent which begins before our range and potentially
+                * intersect it
+                */
                fi = btrfs_item_ptr(leaf, path->slots[0],
                                    struct btrfs_file_extent_item);
                extent_type = btrfs_file_extent_type(leaf, fi);
@@ -1400,19 +1422,28 @@ next_slot:
                                btrfs_file_extent_num_bytes(leaf, fi);
                        disk_num_bytes =
                                btrfs_file_extent_disk_num_bytes(leaf, fi);
+                       /*
+                        * If extent we got ends before our range starts, skip
+                        * to next extent
+                        */
                        if (extent_end <= start) {
                                path->slots[0]++;
                                goto next_slot;
                        }
+                       /* Skip holes */
                        if (disk_bytenr == 0)
                                goto out_check;
+                       /* Skip compressed/encrypted/encoded extents */
                        if (btrfs_file_extent_compression(leaf, fi) ||
                            btrfs_file_extent_encryption(leaf, fi) ||
                            btrfs_file_extent_other_encoding(leaf, fi))
                                goto out_check;
                        /*
-                        * Do the same check as in btrfs_cross_ref_exist but
-                        * without the unnecessary search.
+                        * If extent is created before the last volume's snapshot
+                        * this implies the extent is shared, hence we can't do
+                        * nocow. This is the same check as in
+                        * btrfs_cross_ref_exist but without calling
+                        * btrfs_search_slot.
                         */
                        if (!freespace_inode &&
                            btrfs_file_extent_generation(leaf, fi) <=
@@ -1420,6 +1451,7 @@ next_slot:
                                goto out_check;
                        if (extent_type == BTRFS_FILE_EXTENT_REG && !force)
                                goto out_check;
+                       /* If extent is RO, we must COW it */
                        if (btrfs_extent_readonly(fs_info, disk_bytenr))
                                goto out_check;
                        ret = btrfs_cross_ref_exist(root, ino,
@@ -1443,8 +1475,8 @@ next_slot:
                        disk_bytenr += cur_offset - found_key.offset;
                        num_bytes = min(end + 1, extent_end) - cur_offset;
                        /*
-                        * if there are pending snapshots for this root,
-                        * we fall into common COW way.
+                        * If there are pending snapshots for this root, we
+                        * fall into common COW way
                         */
                        if (!freespace_inode && atomic_read(&root->snapshot_force_cow))
                                goto out_check;
@@ -1480,12 +1512,17 @@ next_slot:
                        BUG();
                }
 out_check:
+               /* Skip extents outside of our requested range */
                if (extent_end <= start) {
                        path->slots[0]++;
                        if (nocow)
                                btrfs_dec_nocow_writers(fs_info, disk_bytenr);
                        goto next_slot;
                }
+               /*
+                * If nocow is false then record the beginning of the range
+                * that needs to be COWed
+                */
                if (!nocow) {
                        if (cow_start == (u64)-1)
                                cow_start = cur_offset;
@@ -1497,6 +1534,12 @@ out_check:
                }
 
                btrfs_release_path(path);
+
+               /*
+                * COW range from cow_start to found_key.offset - 1. As the key
+                * will contain the beginning of the first extent that can be
+                * NOCOW, following one which needs to be COW'ed
+                */
                if (cow_start != (u64)-1) {
                        ret = cow_file_range(inode, locked_page,
                                             cow_start, found_key.offset - 1,