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

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

On 3/11/25 08:42, Benjamin Marzinski wrote:
> 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.

OK. Got it.
As mentioned, I think we really still need to allow switching (or inserting ?)
dm-error. In that case, calling blk_revalidate_disk_zones() should still be fine
as long as there is no change to the zone config, which is true for dm-error. Do
you agree ?


-- 
Damien Le Moal
Western Digital Research