Re: [PATCH v11 08/14] btrfs: add BTRFS_IOC_ENCODED_READ

Nikolay Borisov
2021-10-15T11:45:49Z

On 1.09.21 г. 20:01, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
>
> There are 4 main cases:
>
> 1. Inline extents: we copy the data straight out of the extent buffer.
> 2. Hole/preallocated extents: we fill in zeroes.
> 3. Regular, uncompressed extents: we read the sectors we need directly
> from disk.
> 4. Regular, compressed extents: we read the entire compressed extent
> from disk and indicate what subset of the decompressed extent is in
> the file.
>
> This initial implementation simplifies a few things that can be improved
> in the future:
>
> - We hold the inode lock during the operation.
> - Cases 1, 3, and 4 allocate temporary memory to read into before
> copying out to userspace.
> - We don't do read repair, because it turns out that read repair is
> currently broken for compressed data.
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> fs/btrfs/ctree.h | 4 +
> fs/btrfs/inode.c | 489 +++++++++++++++++++++++++++++++++++++++++++++++
> fs/btrfs/ioctl.c | 111 +++++++++++
> 3 files changed, 604 insertions(+)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index b95ec5fb68d5..cbd7e07c1c34 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3223,6 +3223,10 @@ int btrfs_writepage_cow_fixup(struct page *page);
> void btrfs_writepage_endio_finish_ordered(struct btrfs_inode *inode,
> struct page *page, u64 start,
> u64 end, bool uptodate);
> +struct btrfs_ioctl_encoded_io_args;
> +ssize_t btrfs_encoded_read(struct kiocb *iocb, struct iov_iter *iter,
> + struct btrfs_ioctl_encoded_io_args *encoded);
> +
> extern const struct dentry_operations btrfs_dentry_operations;
> extern const struct iomap_ops btrfs_dio_iomap_ops;
> extern const struct iomap_dio_ops btrfs_dio_ops;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index a87a34f56234..1940f22179ba 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -10500,6 +10500,495 @@ void btrfs_set_range_writeback(struct btrfs_inode *inode, u64 start, u64 end)
> }
> }
>

<snip>

> +
> +static blk_status_t btrfs_encoded_read_check_bio(struct btrfs_io_bio *io_bio)

nit: The gist of this function is to check the csum so how about
renaming it to btrfs_encoded_read_verify_csum

<snip>

> +
> +static void btrfs_encoded_read_endio(struct bio *bio)
> +{
> + struct btrfs_encoded_read_private *priv = bio->bi_private;
> + struct btrfs_io_bio *io_bio = btrfs_io_bio(bio);
> + blk_status_t status;
> +
> + status = btrfs_encoded_read_check_bio(io_bio);
> + if (status) {
> + /*
> + * The memory barrier implied by the atomic_dec_return() here
> + * pairs with the memory barrier implied by the
> + * atomic_dec_return() or io_wait_event() in

nit: I think atomic_dec_return in read_regular_fill_pages is
inconsequential, what we want to ensure is that when the caller of
io_wait_event is woken up by this thread it will observe the
priv->status, which it will, because the atomic-dec_return in this
function has paired with the general barrier interpolated by wait_event.

So for brevity just leave the text to say "by io_wait_event".

> + * btrfs_encoded_read_regular_fill_pages() to ensure that this
> + * write is observed before the load of status in
> + * btrfs_encoded_read_regular_fill_pages().
> + */
> + WRITE_ONCE(priv->status, status);
> + }
> + if (!atomic_dec_return(&priv->pending))
> + wake_up(&priv->wait);
> + btrfs_io_bio_free_csum(io_bio);
> + bio_put(bio);
> +}

<snip>

> @@ -4824,6 +4841,94 @@ static int _btrfs_ioctl_send(struct file *file, void __user *argp, bool compat)
> return ret;
> }
>

<snip>

> + memset((char *)&args + copy_end_kernel, 0,
> + sizeof(args) - copy_end_kernel);

nit: This memset can be eliminated ( in source) by marking args = {};
and just leaving copy from user above.

> +
> + ret = import_iovec(READ, args.iov, args.iovcnt, ARRAY_SIZE(iovstack),
> + &iov, &iter);
> + if (ret < 0)
> + goto out_acct;
> +
> + if (iov_iter_count(&iter) == 0) {
> + ret = 0;
> + goto out_iov;
> + }
> + pos = args.offset;
> + ret = rw_verify_area(READ, file, &pos, args.len);
> + if (ret < 0)
> + goto out_iov;
> +
> + init_sync_kiocb(&kiocb, file);
> + ret = kiocb_set_rw_flags(&kiocb, 0);

This call is a noop due to:
if (!flags)
return 0;

in kiocb_set_rw_flags.

<snip>