Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

[Ard Biesheuvel]

On 14 March 2018 at 16:41, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> On 14 March 2018 at 15:54, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>> On 14 March 2018 at 14:54, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>> On Wed 14-03-18 14:35:12, Ard Biesheuvel wrote:
>>>> On 14 March 2018 at 14:13, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>>> > Does http://lkml.kernel.org/r/20180313224240.25295-1-neelx@xxxxxxxxxx
>>>> > 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.

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.