linux-kernel - Re: [PATCH v4] slob: add size header to all allocations

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Message-ID: <alpine.DEB.2.22.394.2111301621140.6112@gentwo.de>
Date:   Tue, 30 Nov 2021 16:26:09 +0100 (CET)
From:   Christoph Lameter <cl@...two.org>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     David Laight <David.Laight@...LAB.COM>,
        Rustam Kovhaev <rkovhaev@...il.com>,
        "penberg@...nel.org" <penberg@...nel.org>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "corbet@....net" <corbet@....net>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>
Subject: Re: [PATCH v4] slob: add size header to all allocations

On Tue, 30 Nov 2021, Vlastimil Babka wrote:

> So either I missed something or we violate the rule that kmalloc() provides
> blocks where ARCH_KMALLOC_MINALIGN is not just the alignment of their
> beginning but also nothing else touches the N*ARCH_KMALLOC_MINALIGN area
> containing the allocated object.

Indeed.... The DMA API documentation in the kernel states:

Documentation/DMA-API-HOWTO.rst

2) ARCH_KMALLOC_MINALIGN

+   Architectures must ensure that kmalloc'ed buffer is
+   DMA-safe. Drivers and subsystems depend on it. If an architecture
+   isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
+   the CPU cache is identical to data in main memory),
+   ARCH_KMALLOC_MINALIGN must be set so that the memory allocator
+   makes sure that kmalloc'ed buffer doesn't share a cache line with
+   the others. See arch/arm/include/asm/cache.h as an example.


Note that this is only the case for kmalloc. Not for a slab cache setup
separately from the kmalloc array. That is why ARCH_KMALLOC_MINALIGN
exists.