Re: [PATCH] KVM/MIPS32: Export min_low_pfn.

[Ralf Baechle <ralf@linux-mips.org>]

On Fri, May 17, 2013 at 02:42:48PM -0700, David Daney wrote:

> On 05/17/2013 02:06 PM, Sanjay Lal wrote:
> >The KVM module uses the standard MIPS cache management routines, which use min_low_pfn.
> >This creates and indirect dependency, requiring min_low_pfn to be exported.
> >
> >Signed-off-by: Sanjay Lal <sanjayl@kymasys.com>
> >---
> >  arch/mips/kernel/mips_ksyms.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> >diff --git a/arch/mips/kernel/mips_ksyms.c b/arch/mips/kernel/mips_ksyms.c
> >index 6e58e97..0299472 100644
> >--- a/arch/mips/kernel/mips_ksyms.c
> >+++ b/arch/mips/kernel/mips_ksyms.c
> >@@ -14,6 +14,7 @@
> >  #include <linux/mm.h>
> >  #include <asm/uaccess.h>
> >  #include <asm/ftrace.h>
> >+#include <linux/bootmem.h>
> >
> >  extern void *__bzero(void *__s, size_t __count);
> >  extern long __strncpy_from_user_nocheck_asm(char *__to,
> >@@ -60,3 +61,8 @@ EXPORT_SYMBOL(invalid_pte_table);
> >  /* _mcount is defined in arch/mips/kernel/mcount.S */
> >  EXPORT_SYMBOL(_mcount);
> >  #endif
> >+
> >+/* The KVM module uses the standard MIPS cache functions which use
> >+ * min_low_pfn, requiring it to be exported.

The comment is wrong.  min_low_pfn is being referenced by pfn_valid() which
is implemented (simplified for purposes of this discussion) like:

int pfn_valid(unsigned long pfn)
{
	return pfn >= min_low_pfn && pfn < max_mapnr;
}

> >+ */
> >+EXPORT_SYMBOL(min_low_pfn);
> 
> I think I asked this before, but I don't remember the answer:
> 
> Why not put EXPORT_SYMBOL(min_low_pfn) in mm/bootmem.c adjacent to
> where the symbol is defined?
> 
> Cluttering up the kernel with multiple architectures all doing
> architecture specific exports of the same symbol is not a clean way
> of doing things.
> 
> The second time something needs to be done, it should be factored
> out into common code.

pfn_valid() should return 1 if for the pfn passed as the argment a struct
page exists in mem_map[] - this only affects the flatmem case.  However
it is possible that min_low_pfn is bigger than ARCH_PFN_OFFSET in which
case pfn_valid might return 0 even though it should have returned 1.

So I fixed the issue like:

static inline int pfn_valid(unsigned long pfn)
{
	return pfn >= ARCH_PFN_OFFSET && pfn < max_mapnr;
}

frv, ia64, metag, microblaze, score and sh are also implementing pfn_valid
referencing min_low_pfn.

Time to move pfn_valid() to asm-generic, fix the min_low_pfn /
ARCH_PFN_OFFSET issue and while at it, harden it against multiple
evaluation of its arguments?

  Ralf

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