linux-kernel - Re: [RFC PATCH v1] devres: align devres.data strictly only for devm

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Date:   Fri, 20 Dec 2019 18:13:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        Will Deacon <will@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Tejun Heo <tj@...nel.org>, Mark Brown <broonie@...nel.org>
Subject: Re: [RFC PATCH v1] devres: align devres.data strictly only for
 devm_kmalloc()

On Fri, Dec 20, 2019 at 03:01:03PM +0000, Robin Murphy wrote:
> On 2019-12-20 2:06 pm, Peter Zijlstra wrote:

> > 	data = kmalloc(size + sizeof(struct devres), GFP_KERNEL);
> 
> At this point, you'd still need to special-case devm_kmalloc() to ensure
> size is rounded up to the next ARCH_KMALLOC_MINALIGN granule, or you'd go
> back to the original problem of the struct devres fields potentially sharing
> a cache line with the data buffer. That needs to be avoided, because if the
> devres list is modified while the buffer is mapped for noncoherent DMA
> (which could legitimately happen as they are nominally distinct allocations
> with different owners) there's liable to be data corruption one way or the
> other.

Wait up, why are you allowing non-coherent DMA at less than page size
granularity? Is that really sane? Is this really supported behaviour for
devm ?