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Re: [PATCH] hugetlb: Provide a default HPAGE_SHIFT if !CONFIG_HUGETLB_PA

To: David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] hugetlb: Provide a default HPAGE_SHIFT if !CONFIG_HUGETLB_PAGE
From: David Daney <ddaney.cavm@gmail.com>
Date: Wed, 16 Nov 2011 17:00:30 -0800
Cc: "ralf@linux-mips.org" <ralf@linux-mips.org>, "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>, Andrew Morton <akpm@linux-foundation.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, David Daney <david.daney@cavium.com>
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On 11/16/2011 04:08 PM, David Rientjes wrote:
On Wed, 16 Nov 2011, David Daney wrote:

This is required now to get MIPS kernels to compile with
!CONFIG_HUGETLB_PAGE.


Why?

I should have been more specific.  The failure is in Ralf's
mips-for-linux-next branch.


I can't find that branch (it's not in Ralf's tree at git.kernel.org), so
I'm looking at next-20111116.  It doesn't compile for mips for other
reasons related to arch/mips/sgi-ip22/ip22-gio.c.

Ralf's various trees are accessible via linux-mips.org


Wow, we've certainly gone a long way from a patch that appears to be
fixing a problem in Linus' tree based on your commit message.  You're
working from ralf/upstream-sfr.git#mips-for-linux-next from
git.linux-mips.org.


Yes, as I already said, I should have been more specific about this.

Might want to have mentioned that for a patch labeled "hugetlb".

Which is wrong.  MIPS code should not be using HPAGE_SHIFT without
CONFIG_HUGETLB_PAGE and in fact defines it itself for such a configuration
in arch/mips/include/asm/page.h.  The only generic uses are in
page_alloc.c where we need CONFIG_HUGETLB_PAGE_SIZE_VARIABLE, which isn't
available on mips, and in mm/hugetlb.c which requires CONFIG_HUGETLB_PAGE
by way of CONFIG_HUGETLBFS.

So feel free to show the actual compile error this time and I'll suggest a
mips fix for it.

arch/mips/mm/tlb-r4k.c: In function ‘local_flush_tlb_range’:
arch/mips/mm/tlb-r4k.c:129:28: error: ‘HPAGE_SHIFT’ undeclared (first use in
this function)


Do you see where that file has #ifdef's for CONFIG_HUGETLB_PAGE?

Yes I see that.  With git annotate we can even see who wrote it.

The code would be much cleaner if that #ifdef were removed. The author was unaware that a dummy version of pmd_huge() existed in hugetlb.h, making the entire if(pmd_huge()) huge block dead code.

I will prepare a patch that gets rid of the #ifdef CONFIG_HUGETLB_PAGE in that file.

 You need
them here too.  The problem is that is_vm_hugetlb_page() is not #define to
0 for CONFIG_HUGETLB_PAGE=n so the clauses using HPAGE_* aren't compiled
out like the author is expecting.

However, I call B.S. on your reasoning.

It is a well established kernel idiom to supply dummy values for symbols that
are required to be defined in order to form a syntactically correct C program,
but that are known by the programmer to be used only on dead code paths.

This is exactly what we are doing here.

To do otherwise requires that code be cluttered with #ifdefery.


You're wrong,

Ok, then please tell me why:

1) the dummy version of is_vm_hugetlb_page() exists in hugetlb_inline.h?
2) the dummy version of PageHuge() exists in hugetlb.h
3) the dummy version of reset_vma_resv_huge_pages()
4) the dummy version of hugetlb_total_pages() exists in hugetlb.h
5) the dummy version of follow_hugetlb_page() exists in hugetlb.h
6) the dummy version of follow_huge_addr() exists in hugetlb.h
7) the dummy version of copy_hugetlb_page_range() exists in hugetlb.h
8) the dummy version of hugetlb_prefault() exists in hugetlb.h
9) the dummy version of unmap_hugepage_range() exists in hugetlb.h
10) the dummy version of hugetlb_report_meminfo() exists in hugetlb.h
11) the dummy version of hugetlb_report_node_meminfo() exists in hugetlb.h
12) the dummy version of follow_huge_pmd() exists in hugetlb.h
13) the dummy version of follow_huge_pud() exists in hugetlb.h
14) the dummy version of prepare_hugepage_range() exists in hugetlb.h
15) the dummy version of pmd_huge() exists in hugetlb.h
16) the dummy version of pud_huge() exists in hugetlb.h
17) the dummy version of is_hugepage_only_range() exists in hugetlb.h
17) the dummy version of hugetlb_free_pgd_range() exists in hugetlb.h
18) the dummy version of hugetlb_fault() exists in hugetlb.h
19) the dummy version of huge_pte_offset() exists in hugetlb.h
20) the dummy version of dequeue_hwpoisoned_huge_page() exists in hugetlb.h
21) the dummy version of copy_huge_page() exists in hugetlb.h
22) the dummy version of hugetlb_change_protection() exists in hugetlb.h
.
.
.
[Other non HUGETLB_PAGE examples omitted for conciseness]

nobody should be using HPAGE_SHIFT unless they are working
with hugepages and, in fact, that code that placed those "dummy values"
for HPAGE_MASK and HPAGE_SIZE there has since been removed since 2005.

Defining HPAGE_SHIFT to be PAGE_SHIFT is just stupid and doing so may
allow the program to compile but will hide real bugs later on.  In
fact if you merged your patch, it would be a bug since the vma would have
VM_HUGETLB but you'd now be operating on PAGE_SIZE pages!

Like I said, we should be removing those existing definitions of
HPAGE_MASK and HPAGE_SIZE rather than leaving them so someone like you can
come along and pretend there's any legitimacy to them whatsoever and
extend the insanity.

You may not realize it, but changes to include/linux/hugetlb.h go through
Andrew; Ralf won't be merging anything into this generic header file
because of a mips problem.

Indeed, you may not realize that akpm has been on the to or cc list for this entire thread because I explicitly put him there for this exact reason.

Thanks,
David Daney


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