Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

[Michal Hocko]

On Thu 15-03-18 10:17:24, Ard Biesheuvel wrote:
> On 15 March 2018 at 10:14, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Wed 14-03-18 15:54:16, Ard Biesheuvel wrote:
> >> On 14 March 2018 at 14:54, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >> > On Wed 14-03-18 14:35:12, Ard Biesheuvel wrote:
> >> >> On 14 March 2018 at 14:13, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >> >> > Does http://lkml.kernel.org/r/20180313224240.25295-1-neelx@xxxxxxxxxx
> >> >> > fix your issue? From the debugging info you provided it should because
> >> >> > the patch prevents jumping backwards.
> >> >> >
> >> >>
> >> >> The patch does fix the boot hang.
> >> >>
> >> >> But I am concerned that we are papering over a fundamental flaw in
> >> >> memblock_next_valid_pfn().
> >> >
> >> > It seems that memblock_next_valid_pfn is doing the right thing here. It
> >> > is the alignment which moves the pfn back AFAICS. I am not really
> >> > impressed about the original patch either, to be completely honest.
> >> > It just looks awfully tricky. I still didn't manage to wrap my head
> >> > around the original issue though so I do not have much better ideas to
> >> > be honest.
> >>
> >> So first of all, memblock_next_valid_pfn() never refers to its max_pfn
> >> argument, which is odd nut easily fixed.
> >
> > There is a patch to remove that parameter sitting in the mmotm tree.
> >
> >> Then, the whole idea of substracting one so that the pfn++ will
> >> produce the expected value is rather hacky,
> >
> > Absolutely agreed!
> >
> >> But the real problem is that rounding down pfn for the next iteration
> >> is dodgy, because early_pfn_valid() isn't guaranteed to return true
> >> for the rounded down value. I know it is probably fine in reality, but
> >> dodgy as hell.
> >
> > Yes, that is what I meant when saying I was not impressed... I am always
> > nervous when a loop makes jumps back and forth. I _think_ the main
> > problem here is that we try to initialize a partial pageblock even
> > though a part of it is invalid. We should simply ignore struct pages
> > for those pfns. We don't do that and that is mostly because of the
> > disconnect between what the page allocator and early init code refers to
> > as a unit of memory to care about. I do not remember exactly why but I
> > strongly suspect this is mostly a performance optimization on the page
> > allocator side so that we do not have to check each and every pfn. Maybe
> > we should signal partial pageblocks from an early code and drop the
> > optimization in the page allocator init code.
> >
> >> The same applies to the call to early_pfn_in_nid() btw
> >
> > Why?
> 
> By 'the same' I mean it isn't guaranteed to return true for the
> rounded down value *at the API level*. I understand it will be mostly
> fine in reality, but juggling (in)valid PFNs like this is likely to
> end badly.

OK, I see your point now. I can really imagine that sub-pageblocks would
be splitted into different NUMA nodes but that should be really rare.

-- 
Michal Hocko
SUSE Labs