Re: [RFC PATCH 7/7] dm: allow devices to revalidate existing zones

[Benjamin Marzinski <bmarzins@redhat.com>]

On Tue, Mar 11, 2025 at 08:19:29AM +0900, Damien Le Moal wrote:
> On 3/11/25 02:43, Benjamin Marzinski wrote:
> > On Mon, Mar 10, 2025 at 08:59:26AM +0900, Damien Le Moal wrote:
> >> On 3/10/25 07:29, Benjamin Marzinski wrote:
> >>> dm_revalidate_zones() only allowed devices that had no zone resources
> >>> set up to call blk_revalidate_disk_zones(). If the device already had
> >>> zone resources, disk->nr_zones would always equal md->nr_zones so
> >>> dm_revalidate_zones() returned without doing any work. Instead, always
> >>> call blk_revalidate_disk_zones() if you are loading a new zoned table.
> >>>
> >>> However, if the device emulates zone append operations and already has
> >>> zone append emulation resources, the table size cannot change when
> >>> loading a new table. Otherwise, all those resources will be garbage.
> >>>
> >>> If emulated zone append operations are needed and the zone write pointer
> >>> offsets of the new table do not match those of the old table, writes to
> >>> the device will still fail. This patch allows users to safely grow and
> >>> shrink zone devices. But swapping arbitrary zoned tables will still not
> >>> work.
> >>
> >> I do not think that this patch correctly address the shrinking of dm zoned
> >> device: blk_revalidate_disk_zones() will look at a smaller set of zones, which
> >> will leave already hashed zone write plugs outside of that new zone range in the
> >> disk zwplug hash table. disk_revalidate_zone_resources() does not cleanup and
> >> reallocate the hash table if the number of zones shrinks.
> > 
> > This is necessary for DM. There could be plugged bios that are on on
> > these no longer in-range zones.  They will obviously fail when they get
> > reissued, but we need to keep the plugs around so that they *do* get
> > reissued. A cleaner alternative would be to add code to immediately
> > error out all the plugged bios on shrinks, but I was trying to avoid
> > adding a bunch of code to deal with these cases (of course simply
> > disallowing them adds even less code).
> 
> I am confused now :)
> Under the assumption that we do not allow switching to a new table that changes
> the zone configuration (in particualr, there is no grow/shrink of the device),
> then I do not think we have to do anything special for DM.

If we don't allow switching between zoned tables, then we obviously
don't need to make DM call blk_revalidate_disk_zones() on a zoned table
switch. I was just saying that I know that this patch would leave
out-of-range zone plugs behind on a shrink, but that is necessary to
allow shrinking while there could still be outstanding plugged bios
attached to those plugs.

So, if we wanted to allow shrinking, then I think this patch is correct
(although erroring out all the out-of-range bios would be a cleaner
solution). But assuming we don't allow shrinking, then you are correct.
We don't need to do anything special for DM.

-Ben

> 
> 
> -- 
> Damien Le Moal
> Western Digital Research