Re: [LSF/MM TOPIC] guarantee natural alignment for kmalloc()?

[Vlastimil Babka <vbabka@suse.cz>]

On 4/12/19 9:14 AM, James Bottomley wrote:
>> In the session I hope to resolve the question whether this is indeed
>> the right thing to do for all kmalloc() users, without an explicit
>> alignment requests, and if it's worth the potentially worse
>> performance/fragmentation it would impose on a hypothetical new slab
>> implementation for which it wouldn't be optimal to split power-of-two
>> sized pages into power-of-two-sized objects (or whether there are any
>> other downsides).
> 
> I think so.  The question is how aligned?  explicit flushing arch's
> definitely need at least cache line alignment when using kmalloc for
> I/O and if allocations cross cache lines they have serious coherency
> problems.   The question of how much more aligned than this is
> interesting ... I've got to say that the power of two allocator implies
> same alignment as size and we seem to keep growing use cases that
> assume this.

Right, by "natural alignment" I meant exactly that - align to size for
power-of-two sizes.

> I'm not so keen on growing a separate API unless there's
> a really useful mm efficiency in breaking the kmalloc alignment
> assumptions.

I'd argue there's not.

> James
>