Messages in this thread | | | Date | Thu, 15 Mar 2018 12:43:12 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
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On Thu 15-03-18 10:17:24, Ard Biesheuvel wrote: > 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.
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
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