linux-kernel - Re: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS structures and buffers

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Message-ID: <87y1pos08k.fsf@mpe.ellerman.id.au>
Date:   Fri, 27 Jan 2023 22:08:59 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Andrew Donnellan <ajd@...ux.ibm.com>,
        David Laight <David.Laight@...LAB.COM>,
        'Segher Boessenkool' <segher@...nel.crashing.org>
Cc:     "gjoyce@...ux.ibm.com" <gjoyce@...ux.ibm.com>,
        "erichte@...ux.ibm.com" <erichte@...ux.ibm.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "nayna@...ux.ibm.com" <nayna@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "sudhakar@...ux.ibm.com" <sudhakar@...ux.ibm.com>,
        "ruscur@...sell.cc" <ruscur@...sell.cc>,
        "joel@....id.au" <joel@....id.au>,
        "bgray@...ux.ibm.com" <bgray@...ux.ibm.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "gcwilson@...ux.ibm.com" <gcwilson@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS
 structures and buffers

Andrew Donnellan <ajd@...ux.ibm.com> writes:
> On Thu, 2023-01-26 at 17:31 +0000, David Laight wrote:
>> Changing the size to kzalloc() doesn't help.
>> The alignment depends on the allocator and is only required to have
>> a relatively small alignment (ARCH_MINALIGN?) regardless of the size.
>> 
>> IIRC one of the allocators adds a small header to every item.
>> It won't return 16 byte aligned items at all.
>
> I'm relying on the behaviour described in Documentation/core-
> api/memory-allocation.rst:
>
>     The address of a chunk allocated with kmalloc is aligned to at
>     least ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of
>     two, the alignment is also guaranteed to be at least the respective
>     size.
>
> Is this wrong?

I believe it's correct.

For SLAB and SLUB it boils down to:
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/slab_common.c?commit=830b3c68c1fb1e9176028d02ef86f3cf76aa2476#n640

That's where the kmalloc slabs are created (see create_kmalloc_cache())
just below.

If you create your own slab (with kmem_cache_create()) then the
alignment is up to you, so that's why there's no power-of-2 logic in
calculate_alignment().

And SLOB (which we don't use) does something similar:
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/slob.c?commit=830b3c68c1fb1e9176028d02ef86f3cf76aa2476#n493

cheers