linux-kernel - Re: [PATCH 1/6] x86/efi: PFN_ALIGN() _text and

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Message-ID: <20151126111323.GA22863@gmail.com>
Date:	Thu, 26 Nov 2015 12:13:23 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H . Peter Anvin" <hpa@...or.com>, Toshi Kani <toshi.kani@...com>,
	linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	stable@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 1/6] x86/efi: PFN_ALIGN() _text and _end when calculating
 number of pages


* Matt Fleming <matt@...eblueprint.co.uk> wrote:

> On Tue, 24 Nov, at 09:23:23AM, Ingo Molnar wrote:
> > 
> > Didn't we want to do the _end alignment linker script fix instead?
>  
> I think we should do both. This patch is tagged for stable because it
> fixes a bug in the existing code. It's obvious and it's explicit and
> it's much easier to know when someone might want to backport it.
> 
> Changing the linker script which indirectly fixes the above bug is a
> much more subtle solution, with much larger potential for fallout
> because it affects multiple chunks of kernel code.
> 
> > Alignment assumptions are easy to make when symbols are well aligned typically (as 
> > in this case), so we should guarantee the alignment property instead of 
> > complicating the code.
> 
> I don't agree that sprinkling PFN_ALIGN() complicates the code, it's a
> minimal change with a well known kernel idiom. But yes, aligning these
> symbols in the linker script is generally a good idea.
> 
> The two patches are worthwhile, for different reasons; let's do both.

I disagree, this form:

        npages = (_end - _text) >> PAGE_SHIFT;

is a lot clearer to read than:

        npages = (PFN_ALIGN(_end) - PFN_ALIGN(_text)) >> PAGE_SHIFT;

especially once we ensure that _end and _text are page aligned. The latter form 
will only result in cargo-cult carrying over of unnecessary PFN_ALIGN() 
operations.

Section boundaries of the kernel should generally be page aligned, this is useful 
for a number of other reasons as well.

As far as backporting goes, it would generally be _safer_ to backport the linker 
script fix, in case there are other unrealized alignment bugs in the kernel. 
Especially if upstream does the same.

Thanks,

	Ingo
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