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

From: Omar Sandoval <redacted>

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 <redacted>
---
 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)
 	}
 }
 

+
+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

+		 * 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);
+}

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

+
+	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);