[dm-devel] [PATCH 0/2] block: blk_interposer v3

[Sergei Shtepa]

The 02/01/2021 21:50, Mike Snitzer wrote:
> On Mon, Feb 01 2021 at  1:18pm -0500,
> Sergei Shtepa <sergei.shtepa at veeam.com> wrote:
> 
> > The 02/01/2021 18:45, Bart Van Assche wrote:
> > > On 1/28/21 9:12 AM, Sergei Shtepa wrote:
> > > > 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.