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

From: [email-masked] (Will Deacon)
Date: Wed, 16 Nov 2016 16:30:15 +0000

Hi Akashi,

On Mon, Nov 14, 2016 at 02:55:16PM +0900, AKASHI Takahiro wrote:
> On Fri, Nov 11, 2016 at 11:19:04AM +0800, Dennis Chen wrote:
> > On Fri, Nov 11, 2016 at 11:50:50AM +0900, AKASHI Takahiro wrote:
> > > On Thu, Nov 10, 2016 at 05:27:20PM +0000, Will Deacon wrote:
> > > > On Wed, Nov 02, 2016 at 01:51:53PM +0900, AKASHI Takahiro wrote:
> > > > > +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);
> > > > > +
> > > > > +	for (i = start_rgn - 1; i >= 0; i--)
> > > > > +		if (!memblock_is_nomap(&memblock.memory.regions[i]))
> > > > > +			memblock_remove_region(&memblock.memory, i);
> > > > > +
> > > > > +	/* truncate the reserved regions */
> > > > > +	memblock_remove_range(&memblock.reserved, 0, base);
> > > > > +	memblock_remove_range(&memblock.reserved,
> > > > > +			base + size, (phys_addr_t)ULLONG_MAX);
> > > > > +}
> > > > 
> > > > This duplicates a bunch of the logic in memblock_mem_limit_remove_map. Can
> > > > you not implement that in terms of your new, more general, function? e.g.
> > > > by passing base == 0, and size == limit?
> > > 
> > > Obviously it's possible.
> > > I actually talked to Dennis before about merging them,
> > > but he was against my idea.
> > >
> > Oops! I thought we have reached agreement in the thread:http://lists.infradead.org/pipermail/linux-arm-kernel/2016-July/442817.html
> > So feel free to do that as Will'll do
> 
> OK, but I found that the two functions have a bit different semantics
> in clipping memory range, in particular, when the range [base,base+size)
> goes across several regions with a gap.
> (This does not happen in my arm64 kdump, though.)
> That is, 'limit' in memblock_mem_limit_remove_map() means total size of
> available memory, while 'size' in memblock_cap_memory_range() indicates
> the size of _continuous_ memory range.

I thought limit was just a physical address, and then
memblock_mem_limit_remove_map operated on the end of the nearest memblock?
You could leave the __find_max_addr call in memblock_mem_limit_remove_map,
given that I don't think you need/want it for memblock_cap_memory_range.

> So I added an extra argument, exact, to a common function to specify
> distinct behaviors. Confusing? Please see the patch below.

Oh yikes, this certainly wasn't what I had in mind! My observation was
just that memblock_mem_limit_remove_map(limit) does:

  1. memblock_isolate_range(limit - limit+ULLONG_MAX)
  2. memblock_remove_region(all non-nomap regions in the isolated region)
  3. truncate reserved regions to limit

and your memblock_cap_memory_range(base, size) does:

  1. memblock_isolate_range(base - base+size)
  2, memblock_remove_region(all non-nomap regions above and below the
     isolated region)
  3. truncate reserved regions around the isolated region

so, assuming we can invert the isolation in one of the cases, then they
could share the same underlying implementation.

I'm probably just missing something here, because the patch you've ended
up with is far more involved than I anticipated...

Will