Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 15 Mar 2018 06:39:53 +0000 | Subject | Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
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On 15 March 2018 at 02:32, Daniel Vacek <neelx@redhat.com> wrote: > On Wed, Mar 14, 2018 at 6:36 PM, Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: >> On 14 March 2018 at 16:41, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: >>> 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. >> >> 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. >
You only align down after encountering an invalid PFN. If start_pfn itself is not pageblock aligned, how do you initialize the first struct page of the pageblock?
>> 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 >
No, your patch only initializes the first struct page of a pageblock. If the next one is invalid, we will skip to the next valid one.
You are making the assumption that pfn_valid() will return true for all pages in a pageblock if it returns true for one of them, and this does not hold on other architectures.
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