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

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Message-ID: <b016aefff9514ed1ad40620cea6d3b9f@AcuMS.aculab.com>
Date:   Fri, 27 Jan 2023 09:05:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andrew Donnellan' <ajd@...ux.ibm.com>,
        'Segher Boessenkool' <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
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

From: Andrew Donnellan
> Sent: 27 January 2023 03:21
> 
> 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?

The alignment for power of two doesn't match what I've inferred
from reading comments on other patches.

It is true for dma_malloc_coherent() - that does guarantee that a
16k allocate will be aligned on a 16k physical address boundary.

	David

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