linux-kernel - Re: [rfc 0/2] Introducing VmFlags field into smaps output

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Date:	Wed, 24 Oct 2012 01:46:39 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [rfc 0/2] Introducing VmFlags field into smaps output

On Tue, Oct 23, 2012 at 02:30:45PM -0700, Andrew Morton wrote:
> > 
> > The data is encoded in a somewhat awkward two letters mnemonic form, to
> > encourage userspace to be prepared for fields being added or removed in
> > the future.
> > 
> 
> Wow.  This version generates 1k less kernel bloat than v2!  Gee, and I
> only sent that email as a late-night joke ;)

Cool!

> fs/proc/task_mmu.o with neither patch:
>    text    data     bss     dec     hex filename
>   14849     112    5312   20273    4f31 fs/proc/task_mmu.o
> 
> fs/proc/task_mmu.o with the v2 patch:
>   16074     112    5776   21962    55ca fs/proc/task_mmu.o
> 
> fs/proc/task_mmu.o with the v3 patch:
>   15446     112    5368   20926    51be fs/proc/task_mmu.o
> 
> fs/proc/task_mmu.o with the v3 patch and the below fix:
>   15123     112    5352   20587    506b fs/proc/task_mmu.o
> 
> So the delta has gone from 1700 bytes down to 300.  Seems that it pays
> to be anal about these things ;)

LOL ;-)

> Don't forget the `static'!  Without it, the compiler will need to

Yeah, for some reason missed it, thanks!

> construct the array as a temporary on the stack each time the function
> is called - it's just terrible.  (There's no reason why the compiler
> can't insert the static for us as an optimisation, and I think later
> gcc's may have got smarter about this).
> 
> Was there a reason why you added the ".l = " to the initialiser?  My
> gcc is happy without it.

Mine either, just as habbit I would say, thanks.

> Also...  what happens if there's an unrecognised bit set in `flags'? 
> Memory corruption or code skew could cause this.  We emit a couple of
> NULs into the procfs output, which I guess is an OK response to such a
> condition.

That's indeed will produce some nil character on the screen. I guess we
need some designited init here, say

	[0 ... (BITS_PER_LONG-1)] = { {'-', '-'} },
	...
	[ilog2(VM_READ)]	= { {'r', 'd'} },
	...
	and so on, i'll update on top

> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: procfs-add-vmflags-field-in-smaps-output-v3-fix
> 
> make mnemonics[] static, remove unneeded init code, tidy whitespace
> 
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