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

[Matthew Wilcox <willy@infradead.org>]

On Thu, Apr 11, 2019 at 02:52:08PM +0200, Vlastimil Babka 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 this is exactly the kind of discussion that LSFMM is for!  It's
really a whole-system question; is Linux better-off having the flexibility
for allocators to return non-power-of-two aligned memory, or allowing
consumers of the kmalloc API to assume that "sufficiently large" memory
is naturally aligned.

Another possibility that should be considered is introducing a kmalloc()
variant like posix_memalign() that allows for specifying the alignment,
or just kmalloc_naturally_aligned().

And we probably need to reiterate for the benefit of those not following
the discussion that creating a slab cache (which does allow for alignment
to be specified) is impractical for this use case because the actual
allocations are of variable size, but always need to be 512-byte aligned.