On Wed 14-03-18 15:54:16, Ard Biesheuvel wrote: > On 14 March 2018 at 14:54, Michal Hocko <mho...@kernel.org> wrote: > > On Wed 14-03-18 14:35:12, Ard Biesheuvel wrote: > >> On 14 March 2018 at 14:13, Michal Hocko <mho...@kernel.org> wrote: > >> > Does http://lkml.kernel.org/r/20180313224240.25295-1-ne...@redhat.com > >> > 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? -- Michal Hocko SUSE Labs
