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

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Message-ID: <536eb8d0112b44dc90b9b9a7f670e96d@AcuMS.aculab.com>
Date:   Wed, 13 Feb 2019 14:13:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Vineet Gupta <vineet.gupta1@...opsys.com>
CC:     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

From: Peter Zijlstra 
> Sent: 13 February 2019 12:57
...
...
> In the past I've proposed a GCC plugin / checker that would verify the
> alignment requirements against the various allocators.
> 
> For instance:
> 
> struct foo {
> 	spinlock_t a;
> 	int b;
> } __cacheline_aligned;
> 
> struct foo *my_foo = kmalloc(sizeof(struct foo), GFP_KERNEL);
> 
> would result in a warning; because obviously kmalloc (as per
> ARCH_SLAB_MINALIGN) doesn't respect the cacheline alignment of the type.
> 
> Of course; it appears our kmalloc() function definition doesn't even
> have a __malloc attribute, so there's plenty work to be done here.

We could pass the alignment to the allocator by defining something like:

#define do_malloc(x) ((x) = (typeof(*(x)))_do_malloc(sizeof *(x), __alignof__(*(x))))

Although you probably want to compile-time detect alignments that are smaller
than the normal minimal alignment.

If the allocator needs to add a header it would need to use the byte before
the allocated item to find the header.
OTOH adding a header is horrid for page-sized items.

	David

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