Re: [PATCH net-next v6 2/8] ipc, msg: Account msg_msg allocations with GFP_KERNEL_ACCOUNT again

[Harry Yoo <harry@kernel.org>]

On Fri, Oct 09, 2026 at 04:22:03PM +0200, Harry Yoo wrote:
> And now I see the initial kmem_buckets design did not sufficiently
> tackle the question "How this should work when kmem_buckets falls back
> to kmalloc?"
> 
> I suppose the kmem_buckets' abstraction should not be too tightly
> coupled with kmalloc caches. Creating a kmem_buckets should be
> conceptually equivalent to creating a set of caches with speicifc
> slab flags, size, align, useroffset/size. (for variable size allocation).

Just to clarify... I have a few concerns on this area.

1. Complexity.

It's very counter-intuitive to understand how it works when
SLAB_BUCKETS is not compiled. My brain cannot easily process two
different implementations w/ and w/o SLAB_BUCKETS.

A "compatibility layer" allows us to assume certain expectations
and ignore all implementation details when SLAB_BUCKETS is not compiled.

2. Making kmem_buckets tightly coupled with kmalloc's implementation
   details is not reasonable.

Let's say, at some point we make SLAB_BUCKETS always enabled and drop
the config option. Then all kmem_buckets users will now have certain
assumptions inherted from kmalloc (e.g. alignment), which is not
reasonable.

3. __GFP_ACCOUNT and __GFP_DMA should be applied to all allocations
in the slab cache. For kmalloc, these flags are only used to select
the kmalloc cache (which has equivalent SLAB_* flags for __GFP_ACCOUNT
or __GFP_DMA) to serve the allocation.

Specifying it on individual allocations (rather than as SLAB_ flags)
makes it easy to break the assumption.

> When it falls back to kmalloc, kmem_buckets itself should provide a
> compatibility layer when falling back to kmalloc.

For example, calculate_sizes() converts certain SLAB_* flags into
GFP flags (s->allocflags) for allocation. We can have a similar
logic when falling back to kmalloc.

Though current implementation does not allow this and requires
some rework.

-- 
Cheers,
Harry / Hyeonggon