Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages

[David Hildenbrand <david@redhat.com>]

On 25.11.20 13:08, Vlastimil Babka wrote:
> On 11/25/20 6:34 AM, Andrea Arcangeli wrote:
>> Hello,
>>
>> On Mon, Nov 23, 2020 at 02:01:16PM +0100, Vlastimil Babka wrote:
>>> On 11/21/20 8:45 PM, Andrea Arcangeli wrote:
>>>> A corollary issue was fixed in
>>>> 39639000-39814fff : Unknown E820 type
>>>>
>>>> pfn 0x7a200 -> 0x7a200000 min_pfn hit non-RAM:
>>>>
>>>> 7a17b000-7a216fff : Unknown E820 type
>>>
>>> It would be nice to also provide a /proc/zoneinfo and how exactly the 
>>> "zone_spans_pfn" was violated. I assume we end up below zone's 
>>> start_pfn, but is it true?
>>
>> Agreed, I was about to grab that info along with all page struct
>> around the pfn 0x7a200 and phys address 0x7a216fff.
>>
>> # grep -A1 E820 /proc/iomem
>> 7a17b000-7a216fff : Unknown E820 type
>> 7a217000-7bffffff : System RAM
>>
>> DMA      zone_start_pfn 1            zone_end_pfn() 4096         contiguous 1
>> DMA32    zone_start_pfn 4096         zone_end_pfn() 1048576      contiguous 0
>> Normal   zone_start_pfn 1048576      zone_end_pfn() 4715392      contiguous 1
>> Movable  zone_start_pfn 0            zone_end_pfn() 0            contiguous 0
> 
> So the above means that around the "Unknown E820 type" we have:
> 
> pfn 499712 - start of pageblock in ZONE_DMA32
> pfn 500091 - start of the "Unknown E820 type" range
> pfn 500224 - start of another pageblock
> pfn 500246 - end of "Unknown E820 type"
> 
> So this is indeed not a zone boundary issue, but basically a hole not 
> aligned to pageblock boundary and really unexpected.
> We have CONFIG_HOLES_IN_ZONE (that x86 doesn't set) for architectures 
> that do this, and even that config only affects pfn_valid_within(). But 
> here pfn_valid() is true, but the zone/node linkage is unexpected.
> 
>> However the real bug seems that reserved pages have a zero zone_id in
>> the page->flags when it should have the real zone id/nid. The patch I
>> sent earlier to validate highest would only be needed to deal with
>> pfn_valid.
>>
>> Something must have changed more recently than v5.1 that caused the
>> zoneid of reserved pages to be wrong, a possible candidate for the
>> real would be this change below:
>>
>> +               __init_single_page(pfn_to_page(pfn), pfn, 0, 0);
>>
>> Even if it may not be it, at the light of how the reserved page
>> zoneid/nid initialized went wrong, the above line like it's too flakey
>> to stay.
>>
>> It'd be preferable if the pfn_valid fails and the
>> pfn_to_section_nr(pfn) returns an invalid section for the intermediate
>> step. Even better memset 0xff over the whole page struct until the
>> second stage comes around.
>>
>> Whenever pfn_valid is true, it's better that the zoneid/nid is correct
>> all times, otherwise if the second stage fails we end up in a bug with
>> weird side effects.
> 
> Yeah I guess it would be simpler if zoneid/nid was correct for 
> pfn_valid() pfns within a zone's range, even if they are reserved due 
> not not being really usable memory.
> 
> I don't think we want to introduce CONFIG_HOLES_IN_ZONE to x86. If the 
> chosen solution is to make this to a real hole, the hole should be 
> extended to MAX_ORDER_NR_PAGES aligned boundaries.

As we don't punch out pages of the memmap on x86-64, pfn_valid() keeps
working as expected. There is a memmap that can be accessed and that was
initialized. It's really just a matter of how to handle memory holes in
this scenario.

a) Try initializing them to the covering node/zone (I gave one example
   that might be tricky with hotplug)
b) Mark such pages (either special node/zone or pagetype) and make pfn
   walkers ignore these holes. For now, this can only be done via the
   reserved flag.

-- 
Thanks,

David / dhildenb