On Fri, Oct 09, 2026 at 09:39:48PM +0200, Harry Yoo wrote: > 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.
I'd rather make kmem_buckets not have a SLAB_BUCKETS option. :) > 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. Until it's always enabled, I don't see a reasonable way to separate those assumptions. > 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. As in, you don't want a "pass-through" mode for __GFP_DMA? It's a lot of overhead for __GFP_DMA to have multiple providers, so I'd rather allow kmem_buckets to allow pass-through (perhaps explicitly opt-in) where then the other options (e.g. alignment) must match the general kmalloc caches so that pass-through is consistent. > > 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. I think the current series does everything correctly in the sense that nothing is allowing for mismatched fallbacks/passthrus. We could iterate from there once we have a need for this where a non-kmalloc-alignment is wanted, etc? -- Kees Cook
