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

[Ard Biesheuvel]

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.