linux-kernel - Re: [PATCH] ARC: Explicitly set ARCH_SLAB

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Message-ID: <e6138aeb-9ce4-fbfa-4d6a-b76f189a3e47@synopsys.com>
Date:   Wed, 13 Feb 2019 15:23:36 -0800
From:   Vineet Gupta <vineet.gupta1@...opsys.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     David Laight <David.Laight@...LAB.COM>,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Arnd Bergmann <arnd.bergmann@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8

On 2/13/19 4:56 AM, Peter Zijlstra wrote:
>
> Personally I think u64 and company should already force natural
> alignment; but alas.

But there is an ISA/ABI angle here too. e.g. On 32-bit ARC, LDD (load double) is
allowed to take a 32-bit aligned address to load a register pair. Thus all u64
need not be 64-bit aligned (unless attribute aligned 8 etc) hence the relaxation
in ABI (alignment of long long is 4). You could certainly argue that we end up
undoing some of it anyways by defining things like ARCH_KMALLOC_MINALIGN to 8, but
still...

> I though that was part of the reason we have __u64
> and co., so that ABI is invariant to kernel alignment changes.

Apparently not.

>>> I suspect the slab allocator should be returning 8 byte aligned addresses
>>> on all systems.... 
>>
>> why ? As I understand it is still not fool proof against the expected alignment of
>> inner members. There ought to be a better way to enforce all this.
> 
> I agree that for ARC ARCH_SLAB_MINALIGN should be at least 8.

This issue aside, are there other reasons ? Because making it 8 on ARC is just
pending the eventuality for later.