Subject: Re: [PATCH kernel 3/4] powerpc/mm/iommu: Make mm_iommu_new() fail on existing regions

From: David Gibson <[email-masked]>
Date: Wed, 17 Oct 2018 15:45:48 +1100

On Wed, Oct 17, 2018 at 02:34:32PM +1100, Alexey Kardashevskiy wrote:
> 
> 
> On 17/10/2018 12:00, David Gibson wrote:
> > On Mon, Oct 15, 2018 at 08:24:15PM +1100, Alexey Kardashevskiy wrote:
> >> Since we are going to have 2 different preregistering helpers, let's
> >> make it clear that mm_iommu_new() is only for the normal (i.e. not device)
> >> memory and for existing areas mm_iommu_get() should be used instead.
> >>
> >> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> > 
> > I think the idea is sensible.  However (and, yes, this is really an
> > existing bug) - shouldn't we check for a request to add anything
> > overlapping with an existing region, not just one that exactly
> > matches?
> 
> The overlap check is below the changed hunk:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/mm/mmu_context_iommu.c#n150

Ah, right.

In that case can't you just drop this whole if.  I don't see that
there's any use in giving different error codes for "tried to register
exactly a region you registered before" and "tried to register a
region overlapping one you registered before.

> 
> 
> > 
> >> ---
> >>  arch/powerpc/mm/mmu_context_iommu.c | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
> >> index a8c4a3c..839dbce 100644
> >> --- a/arch/powerpc/mm/mmu_context_iommu.c
> >> +++ b/arch/powerpc/mm/mmu_context_iommu.c
> >> @@ -141,8 +141,7 @@ long mm_iommu_new(struct mm_struct *mm, unsigned long ua, unsigned long entries,
> >>  	list_for_each_entry_rcu(mem, &mm->context.iommu_group_mem_list,
> >>  			next) {
> >>  		if ((mem->ua == ua) && (mem->entries == entries)) {
> >> -			++mem->used;
> >> -			*pmem = mem;
> >> +			ret = -EBUSY;
> >>  			goto unlock_exit;
> >>  		}
> >>  
> > 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson