Messages in this thread | | | From | Ard Biesheuvel <> | Date | Wed, 14 Mar 2018 16:41:47 +0000 | Subject | Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
| |
On 14 March 2018 at 15:54, Ard Biesheuvel <ard.biesheuvel@linaro.org> 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. > Then, the whole idea of substracting one so that the pfn++ will > produce the expected value is rather hacky, > > 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. The same applies to the call to early_pfn_in_nid() btw > > So how about something like this (apologies on Gmail's behalf for the > whitespace damage, I can resend it as a proper patch) > > > ---------8<----------- > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 3d974cb2a1a1..b89ca999ee3b 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -5352,28 +5352,29 @@ > * function. They do not exist on hotplugged memory. > */ > if (context != MEMMAP_EARLY) > goto not_early; > > - if (!early_pfn_valid(pfn)) { > + if (!early_pfn_valid(pfn) || !early_pfn_in_nid(pfn, nid)) { > #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP > /* > * Skip to the pfn preceding the next valid one (or > * end_pfn), such that we hit a valid pfn (or end_pfn) > * on our next iteration of the loop. Note that it needs > * to be pageblock aligned even when the region itself > * is not. move_freepages_block() can shift ahead of > * the valid region but still depends on correct page > * metadata. > */ > - pfn = (memblock_next_valid_pfn(pfn, end_pfn) & > - ~(pageblock_nr_pages-1)) - 1; > -#endif > + pfn = memblock_next_valid_pfn(pfn, end_pfn); > + if (pfn >= end_pfn) > + break; > + pfn &= ~(pageblock_nr_pages - 1); > +#else > continue; > +#endif > } > - if (!early_pfn_in_nid(pfn, nid)) > - continue; > if (!update_defer_init(pgdat, pfn, end_pfn, &nr_initialised)) > break; > > #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP > /* > ---------8<----------- > > This ensures that we enter the remainder of the loop with a properly > aligned pfn, rather than tweaking the value of pfn so it assumes the > expected value after 'pfn++'
Um, this does not actually solve the issue. I guess this is due to the fact that a single pageblock size chunk could have both valid and invalid PFNs, and so rounding down the first PFN of the second valid chunk moves you back to the first chunk.
| |