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Re: [PATCH v2 2/2] hugetlb: Provide safer dummy values for HPAGE_MASK an

To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 2/2] hugetlb: Provide safer dummy values for HPAGE_MASK and HPAGE_SIZE
From: David Daney <ddaney.cavm@gmail.com>
Date: Thu, 17 Nov 2011 15:38:42 -0800
Cc: linux-mips@linux-mips.org, ralf@linux-mips.org, linux-kernel@vger.kernel.org, David Daney <david.daney@cavium.com>, David Rientjes <rientjes@google.com>
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On 11/17/2011 03:28 PM, Andrew Morton wrote:
On Thu, 17 Nov 2011 13:57:30 -0800
David Daney<ddaney.cavm@gmail.com>  wrote:

From: David Daney<david.daney@cavium.com>

It was pointed out by David Rientjes that the dummy values for
HPAGE_MASK and HPAGE_SIZE are quite unsafe.  It they are inadvertently
used with !CONFIG_HUGETLB_PAGE, compilation would succeed, but the
resulting code would surly not do anything sensible.

Place BUG() in the these dummy definitions, as we do in similar
circumstances in other places, so any abuse can be easily detected.

Since the only sane place to use these symbols when
!CONFIG_HUGETLB_PAGE is on dead code paths, the BUG() cause any actual
code to be emitted by the compiler.

I assume you meant "omitted" here.

I jumbled it up.  It should read:

... the BUG() will not cause any actual code to be emitted by the compiler. In fact I have verified this on both MIPS64 and x86_64 kernels.

I could re-spin the patch with a corrected changelog if desired.


But I don't think it's true.  Any such code would occur after testing
is_vm_hugetlb_page() or similar, and would have been omitted anyway.


The point being that we are doing:

if (is_vm_hugetlb_page(vma)) {
        /* Do something with HPAGE_MASK*/
} else {
        /* Do something with PAGE_MASK */
}

In the !CONFIG_HUGETLB_PAGE case we have:
static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
{
        return 0;
}

The compiler sees that the usage of the dummy definitions is in a dead code path and nothing is emitted.

--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -111,8 +111,9 @@ static inline void copy_huge_page(struct page *dst, struct 
page *src)
  #define hugetlb_change_protection(vma, address, end, newprot)

  #ifndef HPAGE_MASK
-#define HPAGE_MASK     PAGE_MASK               /* Keep the compiler happy */
-#define HPAGE_SIZE     PAGE_SIZE
+/* Keep the compiler happy with some dummy (but BUGgy) values */

That's a quite poor comment.  This?

I was trying to communicate the presence of the BUG() in the definition. Perhaps it is more confusing than it was before.


--- 
a/include/linux/hugetlb.h~hugetlb-provide-safer-dummy-values-for-hpage_mask-and-hpage_size-fix
+++ a/include/linux/hugetlb.h
@@ -111,7 +111,11 @@ static inline void copy_huge_page(struct
  #define hugetlb_change_protection(vma, address, end, newprot)

  #ifndef HPAGE_MASK
-/* Keep the compiler happy with some dummy (but BUGgy) values */
+/*
+ * HPAGE_MASK and friends are defined if !CONFIG_HUGETLB_PAGE as an
+ * ifdef-avoiding convenience.  However they should never be evaluated at
+ * runtime if !CONFIG_HUGETLB_PAGE.
+ */
  #define HPAGE_MASK    ({BUG(); 0; })
  #define HPAGE_SIZE    ({BUG(); 0; })
  #define HPAGE_SHIFT   ({BUG(); 0; })
_

+#define HPAGE_MASK     ({BUG(); 0; })
+#define HPAGE_SIZE     ({BUG(); 0; })
  #define HPAGE_SHIFT   ({BUG(); 0; })

This change means that HPAGE_* cannot be evaluated at compile time.  So

int foo = HPAGE_SIZE;

outside functions will explode.  I guess that's OK - actually desirable
- as such code shouldn't have been compiled anyway.


The exact point of the patch.

Thanks,
David Daney



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