Re: slab warning: kmem_cache of name 'dm_bufio_buffer' already exists

[Vlastimil Babka <vbabka@suse.cz>]

On 11/8/24 12:13, Hyeonggon Yoo wrote:
> On Fri, Nov 8, 2024 at 6:56 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> On 11/6/24 22:22, Mikulas Patocka wrote:
>> >
>> >
>> > On Wed, 6 Nov 2024, Vlastimil Babka wrote:
>> >
>> >> On 11/6/24 13:05, Mikulas Patocka wrote:
>> >> >
>> >> >
>> >> > On Wed, 6 Nov 2024, Vlastimil Babka wrote:
>> >> >
>> >> >> On 11/6/24 12:19, Mikulas Patocka wrote:
>> >> >> > Hi
>> >> >>
>> >> >> Hi,
>> >> >>
>> >> >> > The commit 4c39529663b93165953ecf9b1a9ea817358dcd06 ("slab: Warn on
>> >> >> > duplicate cache names when DEBUG_VM=y") is causing large number of
>> >> >> > warnings about "dm_bufio_buffer", "dm_bufio_buffer-%u" (and other) device
>> >> >> > mapper caches.
>> >> >>
>> >> >> Hmm wonder why nobody run into this before. We thought the code that would
>> >> >> cause the warning would be all fixed before introducing it, but we missed
>> >> >> some, sorry.
>> >> >>
>> >> >> > I'd like to ask - how to properly fix it?
>> >> >> >
>> >> >> > We create a "dm_bufio_buffer" or "dm_bufio_buffer-%u" cache with every dm
>> >> >> > bufio client. It used to work (and the duplicate caches are merged), but
>> >> >>
>> >> >> Note the merging can be disabled so then it's really several caches with
>> >> >> exactly same name in /proc/slabinfo and inability to create their
>> >> >> sysfs/debugfs directories.
>> >> >
>> >> > Would it be sensible to allow merging caches with the same name and same
>> >> > attributes and only warn if there are caches with the same name and
>> >> > different attributes?
>> >>
>> >> We might consider that.
>> >
>> > That would be good - so that users don't have to write their own slab
>> > cache merge logic.
>>
>> So Pedro, wanna do that? Or Hyeonggon maybe? :)
> 
> IIUC it would be a special slab merging logic if the names and
> attributes match, even if slab merging is disabled. Right?

Yes, I'm thinking of a new SLAB_ flag to make this opt-in and not
accidental. It would need to override even the (debugging etc) flags or
cache-specific that prevent merging, in addition to the global setting.
Instead we'd need to check that everything is exactly the same - flags,
ctor, etc...