Re: [Xenomai] [PATCH v2] ipipe x86 mm: handle huge pages in memory pinning

[Henning Schild <henning.schild@siemens.com>]

On Thu, 28 Jan 2016 11:53:08 +0100
Philippe Gerum <rpm@xenomai.org> wrote:

> On 01/27/2016 02:41 PM, Henning Schild wrote:
> > In 4.1 huge page mapping of io memory was introduced, enable ipipe
> > to handle that when pinning kernel memory.
> > 
> > change that introduced the feature
> > 0f616be120c632c818faaea9adcb8f05a7a8601f
> > 
> > Signed-off-by: Henning Schild <henning.schild@siemens.com>
> > ---
> >  arch/x86/mm/fault.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index fd5bbcc..ca1e75b 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -211,11 +211,15 @@ static inline pmd_t *vmalloc_sync_one(pgd_t
> > *pgd, unsigned long address) pud_k = pud_offset(pgd_k, address);
> >  	if (!pud_present(*pud_k))
> >  		return NULL;
> > +	if (pud_large(*pud))
> > +		return pud_k;
> >  
> >  	pmd = pmd_offset(pud, address);
> >  	pmd_k = pmd_offset(pud_k, address);
> >  	if (!pmd_present(*pmd_k))
> >  		return NULL;
> > +	if (pmd_large(*pmd))
> > +		return pmd_k;
> >  
> >  	if (!pmd_present(*pmd))
> >  		set_pmd(pmd, *pmd_k);
> > @@ -400,6 +404,8 @@ static inline int vmalloc_sync_one(pgd_t *pgd,
> > unsigned long address) 
> >  	if (pud_none(*pud) || pud_page_vaddr(*pud) !=
> > pud_page_vaddr(*pud_ref)) BUG();
> > +	if (pud_large(*pud))
> > +		return 0;
> >  
> >  	pmd = pmd_offset(pud, address);
> >  	pmd_ref = pmd_offset(pud_ref, address);
> > @@ -408,6 +414,8 @@ static inline int vmalloc_sync_one(pgd_t *pgd,
> > unsigned long address) 
> >  	if (pmd_none(*pmd) || pmd_page(*pmd) != pmd_page(*pmd_ref))
> >  		BUG();
> > +	if (pmd_large(*pmd))
> > +		return 0;
> >  
> >  	pte_ref = pte_offset_kernel(pmd_ref, address);
> >  	if (!pte_present(*pte_ref))
> >   
> 
> I'm confused. Assuming the purpose of that patch is to exclude huge
> I/O mappings from pte pinning, why does the changes to the x86_32
> version of the vmalloc_sync_one() helper actually prevent such
> pinning, while the x86_64 version does not?

No the purpose is to include them just like they were before.
vanilla vmalloc_sync_one just must not be called on huge mappings
because it cant handle them. The patch is supposed to make the function
return successfully, stopping early when huge pages are detected.

It changes the implementation of both x86_32 and x86_64.

> Could you explain the logic behind the changes?