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

Goldwyn Rodrigues
2019-09-05T20:42:15Z

On 18:23 05/09, Christoph Hellwig wrote:
> > 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.
>

Well that is how I had started, but it was getting ugly. Besides, I was
moving everything to a new iomap.c file. So, I think I will change the
relevant code in place and then try to move it to iomap.c, depending
on how big the file is..

No wonder I was not getting any reviews from the btrfs developers!

>
> > + 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?

It was a part btrfs code which is carried forward. Yes, we don't need
the page dirtying and uptodate since iomap does it for us.

>
> > + 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?
>

Yes, I should focus on what it should be called eventually as opposed to
the transition.

--
Goldwyn