Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()

From: Peter Zijlstra <peterz@infradead.org>
To: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Will Deacon <will@kernel.org>,
	Marc Gonzalez <marc.w.gonzalez@free.fr>,
	Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Vineet Gupta <Vineet.Gupta1@synopsys.com>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Rafael Wysocki <rjw@rjwysocki.net>,
	LKML <linux-kernel@vger.kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Russell King <rmk+kernel@armlinux.org.uk>,
	Mark Brown <broonie@kernel.org>, Tejun Heo <tj@kernel.org>,
	arcml <linux-snps-arc@lists.infradead.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()
Date: Fri, 20 Dec 2019 21:23:46 +0100	[thread overview]
Message-ID: <20191220202346.GT2827@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CY4PR1201MB012011E554FC69F7B074B7E2A12D0@CY4PR1201MB0120.namprd12.prod.outlook.com>

On Fri, Dec 20, 2019 at 07:32:16PM +0000, Alexey Brodkin wrote:

> Well it somehow used to work for quite some time now with the data-buffer
> being allocated with 4 words offset (which is 16 bytes for 32-bit platform

3 words, devres_node is 3 words.

Which is exactly why we had to change it, the odd alignment caused ARC
to explode.

> and 32 for 64-bit which is still much less than mentioned 128 bytes).
> Or we just never managed to identify those rare cases when data corruption
> really happened?

The races are rather rare methinks, you'd have to get a list-op
concurrently with a DMA.

If you get the list corrupted, I'm thinking the crash is fairly likely,
albeit really difficuly to debug.

> > No matter which way round you allocate devres and data, by necessity
> > they're always going to consume the same total amount of memory.
> 
> So then the next option I guess is to separate meta-data from data buffers
> completely. Are there any reasons to not do that

Dunno, should work just fine I think.

> other than the hack we're
> discussing here (meta-data in the beginning of the buffer) used to work OK-ish?

If meta-data at the beginngin used to work, I don't see why meta-data at
the end wouldn't work equally well. They'd be equally broken.

But I'm still flabbergasted at the fact that they're doing non-coherent
DMA to kmalloc memory, I thought we had a DMA api for that, with a
special allocator and everything (but what do I know, I've never used
that).

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