linux-kernel - Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE

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Date:	Thu, 17 Nov 2011 15:52:28 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	David Daney <ddaney.cavm@...il.com>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Daney <david.daney@...ium.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Robin Holt <holt@....com>
Subject: Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE_SIZE

On 11/17/2011 03:44 PM, David Rientjes wrote:
> On Thu, 17 Nov 2011, Andrew Morton wrote:
>
>>> So, just remove the dummy and dangerous definitions since they are no
>>> longer needed and reveals the correct dependencies.  Tested on
>>> architectures using the definitions with allyesconfig: x86 (even with
>>> thp), hppa, mips, powerpc, s390, sh3, sh4, sparc, and sparc64, and
>>> with defconfig on ia64.
>>
>> How could arch/mips/mm/tlb-r4k.c:local_flush_tlb_range() compile OK
>> with this change?
>>
>
> This was tested on Linus' tree, not on Ralf's linux-next tree.  All uses
> of HPAGE_* are protected by CONFIG_HUGETLB_PAGE as it appropriately should
> be in Linus' tree in that file.
>
>> What that function is doing looks reasonable to me.  Why fill the poor
>> thing with an ifdef mess?
>>
>> otoh, catching mistakes is good too.  Doing it at runtime as David
>> proposes is OK.
>>
>
> Nobody else needs it other than Ralf's pending change, and you're
> suggesting we need them in a generic header file when any sane arch that
> uses hugepages (all of them, in the current tree) declares these
> themselves in arch/*/include/asm/page.h where it's supposed to be done?
>
> Why on earth do we have CONFIG_HUGETLB_PAGE for at all, then?  To catch
> code that's operating on hugepages when our kernel doesn't support it.
> I'd much rather break the build than get a runtime BUG() because we want
> to avoid an #ifdef or actually write well-written code like every other
> arch has!  Panicking the code to find errors like this is just insanity.
>

A counter argument would be:

There are hundreds of places in the kernel where dummy definitions are 
selected by !CONFIG_* so that we can do:

    if (test_something()) {
       do_one_thing();
    } else {
       do_the_other_thing();
    }


Rather than:

#ifdef CONFIG_SOMETHING
    if (test_something()) {
       do_one_thing();
    } else
#else
    {
       do_the_other_thing();
    }



We even do this all over the place with dummy definitions selected by 
CONFIG_HUGETLB_PAGE, What exactly makes HPAGE_MASK special and not the 
hundreds of other similar situations?

David Daney

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