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

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Date:   Tue, 30 Nov 2021 10:41:15 +0100 (CET)
From:   Christoph Lameter <cl@...two.org>
To:     Rustam Kovhaev <rkovhaev@...il.com>
cc:     David Laight <David.Laight@...lab.com>,
        'Vlastimil Babka' <vbabka@...e.cz>,
        "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 Mon, 29 Nov 2021, Rustam Kovhaev wrote:

> > I'm not sure that helps.
> > The header can't share a cache line with the previous item (because it
> > might be mapped for DMA) so will always take a full cache line.
>
> I thought that DMA API allocates buffers that are larger than page size.
> DMA pool seems to be able to give out smaller buffers, but underneath it
> seems to be calling page allocator.
> The SLOB objects that have this header are all less than page size, and
> they cannot end up in DMA code paths, or can they?

kmalloc slab allocations must return dmaable memory. If the underlying
hardware can only dma to a cache line border then all objects must be
aligned that way.