Re: patch "devres: Align data[] to ARCH_KMALLOC_MINALIGN" added to driver-core-linus

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Fri, Feb 08, 2019 at 07:13:39AM +0000, Alexey Brodkin wrote:
> Hi Greg,
> 
> > -----Original Message-----
> > From: gregkh@linuxfoundation.org <gregkh@linuxfoundation.org>
> > Sent: Thursday, February 7, 2019 6:52 PM
> > To: Alexey Brodkin <alexey.brodkin@synopsys.com>
> > Cc: David.Laight@ACULAB.COM; geert@linux-m68k.org; peterz@infradead.org; stable@vger.kernel.org;
> > tglx@linutronix.de; will.deacon@arm.com; Vineet Gupta <vineet.gupta1@synopsys.com>; linux-snps-
> > arc@lists.infradead.org
> > Subject: Re: patch "devres: Align data[] to ARCH_KMALLOC_MINALIGN" added to driver-core-linus
> 
> [snip]
>  
> > Ah, I was waiting to see if you would notice :)
> 
> Well I was just patiently waiting as I guess there's a long queue
> of patches to deal with in your inbox :)
> 
> > See this question from Linus about this patch:
> > 	https://lore.kernel.org/lkml/CAHk-=wj3Q7CkMQYwfZSsqUTqkEhNwVGrRbCwe7AVJ70S8i5sWw@mail.gmail.com/
> 
> I didn't see that. Though I intentionally sent my patch to most if not all
> arch maintainers so they might share their concerns... but IIRC nobody ever
> replied with either concerns or acks.
> 
> Also I do agree that it's a trade-off between:
>  1. Predictability
>     I was completely sure devm-allocated buffer is the same as anything kmalloced
>     except some meta-data stored _separately_ and so supposed alignment
>     should match as well... but how wrong that feeling was.
> 
>  2. Optimization
>     Indeed it's so sweet when both devm "meta-data" and real small buffer fit
>     into 1 cache line.
>  
> > I figured that you all did this for a good reason, and wasting that much
> > space was going to be ok.  But, I wanted to be sure, so if you never
> > noticed it, I figured it was not that pressing of an issue.
> 
> It's not super pressing because:
>  1. Fortunately [or unfortunately] this problem happens only in pretty rare cases
>     like that Etnaviv driver where I first caught it.
> 
>  2. There's a solution and affected parties may apply known patch locally.
> 
> > Anyway, is this really needed to be backported?
> 
> For us poor ARC developers and users it's really needed as our tools ABI
> sets 32-bit alignment for 64-bit types. See that's the same optimization -
> why wasting precious bytes on useless holes - let's pack data tighter :)
> 
> So having that fix at least in the most recent LTS (i.e. 4.19) would be really good.
> As for older kernels I think for now we may not touch them as indeed change is
> quite intrusive.

Ok, that sounds reasonable.  I'll go add the patch there and see if the
ARM64 people even notice :)

thanks,

greg k-h