Re: [PATCH 04/15] btrfs: Add a simple buffered iomap write

Christoph Hellwig
2019-09-05T16:23:50Z

> Most of the code is "inspired" by
> fs/btrfs/file.c. To keep the size small, all removals are in
> following patches.

Wouldn't it be better to massage the existing code into a form where you
can fairly easily switch over to iomap? That is start refactoring the
code into helpers that are mostly reusable and then just have a patch
switching over. That helps reviewing what actually changes. It's
also what we did for XFS.

> + if (!ordered) {
> + break;
> + }

No need for the braces.

> +static void btrfs_buffered_page_done(struct inode *inode, loff_t pos,
> + unsigned copied, struct page *page,
> + struct iomap *iomap)
> +{
> + if (!page)
> + return;
> + SetPageUptodate(page);
> + ClearPageChecked(page);
> + set_page_dirty(page);
> + get_page(page);
> +}

Thіs looks really strange. Can you explain me why you need the
manual dirtying and SetPageUptodate, and an additional page refcount
here?

> + if (ret < 0) {
> + /*
> + * Space allocation failed. Let's check if we can
> + * continue I/O without allocations
> + */
> + if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> + BTRFS_INODE_PREALLOC)) &&
> + check_can_nocow(BTRFS_I(inode), pos,
> + &write_bytes) > 0) {
> + bi->nocow = true;
> + /*
> + * our prealloc extent may be smaller than
> + * write_bytes, so scale down.
> + */
> + bi->reserved_bytes = round_up(write_bytes +
> + sector_offset,
> + fs_info->sectorsize);
> + } else {
> + goto error;
> + }

Maybe move the goto into the inverted if so you can reduce indentation
by one level?

> + } else {
> + u64 __pos = round_down(pos + written, fs_info->sectorsize);

Line over > 80 characters, and a somewhat odd variabke name.

> + if (bi->nocow) {
> + struct btrfs_root *root = BTRFS_I(inode)->root;
> + btrfs_end_write_no_snapshotting(root);
> + if (written > 0) {
> + u64 start = round_down(pos, fs_info->sectorsize);
> + u64 end = round_up(pos + written, fs_info->sectorsize) - 1;

Line > 80 chars.

> + set_extent_bit(&BTRFS_I(inode)->io_tree, start, end,
> + EXTENT_NORESERVE, NULL, NULL, GFP_NOFS);
> + }
> +
> + }
> + btrfs_delalloc_release_extents(BTRFS_I(inode), bi->reserved_bytes,
> + true);
> +
> + if (written < fs_info->nodesize)
> + btrfs_btree_balance_dirty(fs_info);
> +
> + extent_changeset_free(bi->data_reserved);
> + kfree(bi);
> + return ret;
> +}

> +static const struct iomap_ops btrfs_buffered_iomap_ops = {
> + .iomap_begin = btrfs_buffered_iomap_begin,
> + .iomap_end = btrfs_buffered_iomap_end,
> +};
> +
> +size_t btrfs_buffered_iomap_write(struct kiocb *iocb, struct iov_iter *from)
> +{
> + ssize_t written;
> + struct inode *inode = file_inode(iocb->ki_filp);
> + written = iomap_file_buffered_write(iocb, from, &btrfs_buffered_iomap_ops);

no empty line after the variable declarations? Also this adds a > 80
character line.

> + if (written > 0)
> + iocb->ki_pos += written;

I wonder if we should fold the ki_pos update into
iomap_file_buffered_write. But the patch looks fine even without that.

Also any reason to not name this function btrfs_buffered_write and
keep it in file.c with the rest of the write code?