Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA

[Will Deacon]

On Thu, Jan 05, 2017 at 12:24:07PM +0100, Robert Richter wrote:
> On 04.01.17 14:02:23, Will Deacon wrote:
> > Using early_pfn_valid feels like a bodge to me, since having pfn_valid
> > return false for something that early_pfn_valid says is valid (and is
> > therefore initialised in the memmap) makes the NOMAP semantics even more
> > confusing.
> 
> The concern I have had with HOLES_IN_ZONE is that it enables
> pfn_valid_within() for arm64. This means that each pfn of a section is
> checked which is done only once for the section otherwise. With up to
> 2^18 pages per section we traverse the memblock list by that factor
> more often. There could be a performance regression.

There could be, but we're trying to fix a bug here. I wouldn't have
thought that walking over pfns like that is done very often.

> I haven't numbers yet, since the fix causes another kernel crash. And,
> this is the next problem I have. The crash doesn't happen otherwise. So,
> either it uncovers another bug or the fix is incomplete. Though the
> changes look like it should work. This needs more investigation.

I really can't see how the fix causes a crash, and I couldn't reproduce
it on any of my boards, nor could any of the Linaro folk afaik. Are you
definitely running mainline with just these two patches from Ard?

Will