[PATCH v27 1/9] memblock: add memblock_cap_memory_range()

[takahiro.akashi@linaro.org (AKASHI Takahiro)]

James,

On Thu, Nov 17, 2016 at 06:00:58PM +0000, James Morse wrote:
> Hi Will, Akashi,
> 
> On 17/11/16 11:19, Will Deacon wrote:
> > It looks much better, thanks! Just one question below.
> > 
> 
> > On Thu, Nov 17, 2016 at 02:34:24PM +0900, AKASHI Takahiro wrote:
> >> diff --git a/mm/memblock.c b/mm/memblock.c
> >> index 7608bc3..fea1688 100644
> >> --- a/mm/memblock.c
> >> +++ b/mm/memblock.c
> >> @@ -1514,11 +1514,37 @@ void __init memblock_enforce_memory_limit(phys_addr_t limit)
> >>  			      (phys_addr_t)ULLONG_MAX);
> >>  }
> >>  
> >> +void __init memblock_cap_memory_range(phys_addr_t base, phys_addr_t size)
> >> +{
> >> +	int start_rgn, end_rgn;
> >> +	int i, ret;
> >> +
> >> +	if (!size)
> >> +		return;
> >> +
> >> +	ret = memblock_isolate_range(&memblock.memory, base, size,
> >> +						&start_rgn, &end_rgn);
> >> +	if (ret)
> >> +		return;
> >> +
> >> +	/* remove all the MAP regions */
> >> +	for (i = memblock.memory.cnt - 1; i >= end_rgn; i--)
> >> +		if (!memblock_is_nomap(&memblock.memory.regions[i]))
> >> +			memblock_remove_region(&memblock.memory, i);
> > 
> > In the case that we have only one, giant memblock that covers base all
> > of base + size, can't we end up with start_rgn = end_rgn = 0? In which
> 
> Can this happen? If we only have one memblock that exactly spans
> base:(base+size), memblock_isolate_range() will hit the '@rgn is fully
> contained, record it' code and set start_rgn=0,end_rgn=1. (rbase==base,
> rend==end). We only go round the loop once.
> 
> If we only have one memblock that is bigger than base:(base+size) we end up with
> three regions, start_rgn=1,end_rgn=2. The trickery here is the '@rgn intersects
> from above' code decreases the loop counter so we process the same entry twice,
> hitting '@rgn is fully contained, record it' the second time round... so we go
> round the loop four times.

Thank you for your observation.

> I can't see how we hit the:
> > 	if (rbase >= end)
> > 		break;
> > 	if (rend <= base)
> > 		continue;
> 
> code in either case...

Right. So 'end_rgn' will never be expected to be 0 as far as some
intersection exists.

-Takahiro AKASHI

> 
> 
> Thanks,
> 
> James
> 
> 
> > case, we'd end up accidentally removing the map regions here.
> > 
> > The existing code:
> > 
> >> -	/* remove all the MAP regions above the limit */
> >> -	for (i = end_rgn - 1; i >= start_rgn; i--) {
> >> -		if (!memblock_is_nomap(&type->regions[i]))
> >> -			memblock_remove_region(type, i);
> >> -	}
> > 
> > seems to handle this.