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

[Peter Zijlstra <peterz@infradead.org>]

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).