On Thu, Sep 20, 2018 at 07:15:41PM +0200, David Sterba wrote:

On Wed, Sep 19, 2018 at 10:02:17PM -0700, Omar Sandoval wrote:

quoted

From: Omar Sandoval <redacted>

Btrfs has not allowed swap files since commit 35054394c4b3 ("Btrfs: stop
providing a bmap operation to avoid swapfile corruptions"). However, now
that the proper restrictions are in place, Btrfs can support swap files
through the swap file a_ops, similar to iomap in commit 67482129cdab
("iomap: add a swapfile activation function").

For Btrfs, activation needs to make sure that the file can be used as a
swap file, which currently means that it must be fully allocated as
nocow with no compression on one device. It must also do the proper
tracking so that ioctls will not interfere with the swap file.
Deactivation clears this tracking.

Signed-off-by: Omar Sandoval <redacted>
---
 fs/btrfs/inode.c | 317 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 317 insertions(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 3ea5339603cf..0586285b1d9f 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c

[snip]

quoted

+static int btrfs_swap_activate(struct swap_info_struct *sis, struct file *file,
+			       sector_t *span)
+{
+	struct inode *inode = file_inode(file);
+	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
+	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
+	struct extent_state *cached_state = NULL;
+	struct extent_map *em = NULL;
+	struct btrfs_device *device = NULL;
+	struct btrfs_swap_info bsi = {
+		.lowest_ppage = (sector_t)-1ULL,
+	};
+	int ret = 0;
+	u64 isize = inode->i_size;
+	u64 start;
+
+	/*
+	 * If the swap file was just created, make sure delalloc is done. If the
+	 * file changes again after this, the user is doing something stupid and
+	 * we don't really care.
+	 */
+	ret = btrfs_wait_ordered_range(inode, 0, (u64)-1);
+	if (ret)
+		return ret;
+
+	/*
+	 * The inode is locked, so these flags won't change after we check them.
+	 */
+	if (BTRFS_I(inode)->flags & BTRFS_INODE_COMPRESS) {
+		btrfs_info(fs_info, "swapfile must not be compressed");
+		return -EINVAL;
+	}
+	if (!(BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW)) {
+		btrfs_info(fs_info, "swapfile must not be copy-on-write");
+		return -EINVAL;
+	}

I wonder if we should also explicitly check for the checkums flag, ie.
that NODATASUM is present. Right now it's bound to NODATACOW, but as
with other sanity checks, it does not hurt to have it here.

quoted

+
+	/*
+	 * Balance or device remove/replace/resize can move stuff around from
+	 * under us. The EXCL_OP flag makes sure they aren't running/won't run
+	 * concurrently while we are mapping the swap extents, and
+	 * fs_info->swapfile_pins prevents them from running while the swap file
+	 * is active and moving the extents. Note that this also prevents a
+	 * concurrent device add which isn't actually necessary, but it's not
+	 * really worth the trouble to allow it.
+	 */
+	if (test_and_set_bit(BTRFS_FS_EXCL_OP, &fs_info->flags))
+		return -EBUSY;

This could be also accompanied by a message, "why does not my swapfile
activate?" -> "there's an exclusive operation running". I've checked if
there are similar messages for the other exclusive ops. There are.

Sounds good. I addressed all of your comments and pushed to
https://github.com/osandov/linux/tree/btrfs-swap. The only thing I
didn't change was the btrfs_info about not being able to relocate an
active swapfile. I think it makes sense as btrfs_info since we already
log every block group we are relocating as info (see
describe_relocation()).