On Wed, Mar 14, 2018 at 6:36 PM, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: > On 14 March 2018 at 16:41, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: >> On 14 March 2018 at 15:54, Ard Biesheuvel <ard.biesheu...@linaro.org> 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. >>> 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. > > OK, so the original patch attempted to ensure that of each pageblock, > at least the first struct page gets initialized, even though the PFN > may not be valid. Unfortunately, this code is not complete, given that > start_pfn itself may be misaligned, and so the issue it attempts to > solve may still occur.
You're wrong here. > Then, I think it is absolutely dodgy to settle for only initializing > the first struct page, rather than all of them, only because a > specific VM_BUG_ON() references the flag field of the first struct > page. > IMO, we should fix this by initializing all struct page entries for > each pageblock sized chunk that has any valid PFNs. That's precisely what my patch does. At least with CONFIG_HAVE_ARCH_PFN_VALID disabled. And it looks only arm implements arch pfn_valid() which I was not testing with and I am not sure it's correct. Check my other email --nX
