Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 15 Mar 2018 10:17:24 +0000 | Subject | Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
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On 15 March 2018 at 10:14, Michal Hocko <mhocko@kernel.org> wrote: > On Wed 14-03-18 15:54:16, Ard Biesheuvel wrote: >> On 14 March 2018 at 14:54, Michal Hocko <mhocko@kernel.org> wrote: >> > On Wed 14-03-18 14:35:12, Ard Biesheuvel wrote: >> >> On 14 March 2018 at 14:13, Michal Hocko <mhocko@kernel.org> wrote: >> >> > Does http://lkml.kernel.org/r/20180313224240.25295-1-neelx@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?
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.
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