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

[Damien Le Moal <dlemoal@kernel.org>]

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.


-- 
Damien Le Moal
Western Digital Research