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	Thu, 15 Jan 2026 02:08:51 +0000
Date: Thu, 15 Jan 2026 02:08:50 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: "Christoph Lameter (Ampere)" <cl@gentwo.org>
Cc: linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>,
	Harry Yoo <harry.yoo@oracle.com>, linux-fsdevel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	Mateusz Guzik <mguzik@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 00/15] kmem_cache instances with static storage
 duration
Message-ID: <20260115020850.GX3634291@ZenIV>
References: <20260110040217.1927971-1-viro@zeniv.linux.org.uk>
 <0727b5a1-078f-0055-fc52-61b80bc5d59e@gentwo.org>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Wed, Jan 14, 2026 at 04:46:04PM -0800, Christoph Lameter (Ampere) wrote:
> On Sat, 10 Jan 2026, Al Viro wrote:
> 
> > 1) as it is, struct kmem_cache is opaque for anything outside of a few
> > files in mm/*; that avoids serious headache with header dependencies,
> > etc., and it's not something we want to lose.  Solution: struct
> > kmem_cache_opaque, with the size and alignment identical to struct
> > kmem_cache.  Calculation of size and alignment can be done via the same
> > mechanism we use for asm-offsets.h and rq-offsets.h, with build-time
> > check for mismatches.  With that done, we get an opaque type defined in
> > linux/slab-static.h that can be used for declaring those caches.
> > In linux/slab.h we add a forward declaration of kmem_cache_opaque +
> > helper (to_kmem_cache()) converting a pointer to kmem_cache_opaque
> > into pointer to kmem_cache.
> 
> Hmmm. A new kernel infrastructure feature: Opaque objects
> 
> Would that an deserve a separate abstraction so it is usable by other
> subsystems?

*shrug*

Probably could be done, but I don't see many applications for that.
Note that in this case objects are either of "never destroyed at all"
sort or "never destroyed until rmmod" one, and the latter already
requires a pretty careful handling.

If it's dynamically allocated, we have much more straightforward
mechanisms - see e.g. struct mount vs. struct vfsmount, where most
of the containing object is opaque for everyone outside of several
files in fs/*.c and the public part is embedded into it.

I'm not saying that no other similar cases exist, but until somebody
comes up with other examples...