Re: [PATCH 0/2] block: blk_interposer v3

The 02/01/2021 21:50, Mike Snitzer wrote:

On Mon, Feb 01 2021 at  1:18pm -0500,
Sergei Shtepa [off-list ref] wrote:

quoted

The 02/01/2021 18:45, Bart Van Assche wrote:

quoted

On 1/28/21 9:12 AM, Sergei Shtepa wrote:

quoted

I`m ready to suggest the blk_interposer again.
blk_interposer allows to intercept bio requests, remap bio to
another devices or add new bios.

This version has support from device mapper.

For the dm-linear device creation command, the `noexcl` parameter
has been added, which allows to open block devices without
FMODE_EXCL mode. It allows to create dm-linear device on a block
device with an already mounted file system.
The new ioctl DM_DEV_REMAP allows to enable and disable bio
interception.

Thus, it is possible to add the dm-device to the block layer stack
without reconfiguring and rebooting.

What functionality does this driver provide that is not yet available in 
a RAID level 1 (mirroring) driver + a custom dm driver? My understanding 
is that there are already two RAID level 1 drivers in the kernel tree 
and that both driver support sending bio's to two different block devices.

Thanks,

Bart.

Hi Bart.

The proposed patch is not realy aimed at RAID1.

Creating a new dm device in the non-FMODE_EXCL mode and then remaping bio
requests from the regular block device to the new DM device using
the blk_interposer will allow to use device mapper for regular devices.
For dm-linear, there is not much benefit from using blk_interposer.
This is a good and illustrative example. Later, using blk-interposer,
it will be possible to connect the dm-cache "on the fly" without having
to reboot and/or reconfigure.
My intention is to let users use dm-snap to create snapshots of any device.
blk-interposer will allow to add new features to Device Mapper.

As per Daniel's advice I want to add a documentation, I'm working on it now.
The documentation will also contain a description of new features that
blk_interposer will add to Device Mapper

More Documentation is fine, but the code needs to be improved and fully
formed before you start trying to polish with Documentation --
definitely don't put time to Documentation that is speculative!

You'd do well to focus on an implementation that doesn't require an
extra clone if interposed device will use DM (DM core already handles
cloning all incoming bios).

Mike

Hi Mike.

I agree that quality code is more important than documentation.
And I think this code has already solved the problem of a bio extra cloning.

+static void dm_remap_fn(void *context, struct dm_rb_range *node, struct bio *bio)
+{
+	struct mapped_device *md = context;
+
+	/* Set acceptor device. */
+	bio->bi_disk = md->disk;
+
+	/* Remap disks offset */
+	bio->bi_iter.bi_sector -= node->start;
+
+	/*
+	 * bio should be resubmitted.
+	 * We can just add bio to bio_list of the current process.
+	 * current->bio_list must be initialized when this function is called.
+	 * If call submit_bio_noacct(), the bio will be checked twice.
+	 */
+	BUG_ON(!current->bio_list);
+	bio_list_add(&current->bio_list[0], bio);
+}

-- 
Sergei Shtepa
Veeam Software developer.