Re: [PATCH] mm/compaction: Fix the incorrect hole in fast_isolate_freepages()

[David Hildenbrand]

On 26.05.20 10:45, Baoquan He wrote:
> On 05/22/20 at 05:20pm, Mike Rapoport wrote:
>> Hello Baoquan,
>>
>> On Fri, May 22, 2020 at 03:25:24PM +0800, Baoquan He wrote:
>>> On 05/22/20 at 03:01pm, Baoquan He wrote:
>>>>
>>>> So let's add these unavailable ranges into memblock and reserve them
>>>> in init_unavailable_range() instead. With this change, they will be added
>>>> into appropriate node and zone in memmap_init(), and initialized in
>>>> reserve_bootmem_region() just like any other memblock reserved regions.
>>>
>>> Seems this is not right. They can't get nid in init_unavailable_range().
>>> Adding e820 ranges may let them get nid. But the hole range won't be
>>> added to memblock, and still has the issue.
>>>
>>> Nack this one for now, still considering.
>>
>> Why won't we add  the e820 reserved ranges to memblock.memory during
>> early boot as I suggested?
>>
>> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
>> index c5399e80c59c..b0940c618ed9 100644
>> --- a/arch/x86/kernel/e820.c
>> +++ b/arch/x86/kernel/e820.c
>> @@ -1301,8 +1301,11 @@ void __init e820__memblock_setup(void)
>>  		if (end != (resource_size_t)end)
>>  			continue;
>>  
>> -		if (entry->type == E820_TYPE_SOFT_RESERVED)
>> +		if (entry->type == E820_TYPE_SOFT_RESERVED ||
>> +		    entry->type == E820_TYPE_RESERVED) {
>> +			memblock_add(entry->addr, entry->size);
>>  			memblock_reserve(entry->addr, entry->size);
>> +		}
>>  
>>  		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
>>  			continue;
>>
>> The setting of node later  in numa_init() will assign the proper node
>> for these regions as it does for the usable memory.
> 
> Yes, if it's only related to e820 reserved region, this truly works.
> 
> However, it also has ACPI table regions. That's why I changed to call
> the problematic area as firmware reserved ranges later.
> 
> Bisides, you can see below line, there's another reserved region which only
> occupies one page in one memory seciton. If adding to memblock.memory, we also
> will build struct mem_section and the relevant struct pages for the whole
> section. And then the holes around that page will be added and initialized in
> init_unavailable_mem(). numa_init() will assign proper node for memblock.memory
> and memblock.reserved, but won't assign proper node for the holes.
> 
> ~~~
> [    0.000000] BIOS-e820: [mem 0x00000000fed80000-0x00000000fed80fff] reserved
> ~~~
> 
> So I still think we should not add firmware reserved range into
> memblock for fixing this issue.
> 
> And, the fix in the original patch seems necessary. You can see in
> compaction code, the migration source is chosen from LRU pages or
> movable pages, the migration target has to be got from Buddy. However,
> only the min_pfn in fast_isolate_freepages(), it's calculated by
> distance between cc->free_pfn - cc->migrate_pfn, we can't guarantee it's
> safe, then use it as the target to handle.
> 
> Hi David,
> 
> Meanwhile, I checked the history of init_unavailable_mem(). Till below
> commit From David, the unavailable ranges began to be added to zone 0
> and node 0. Before that, we only zero the struct page of unavailable
> ranges and mark it as Reserved. Am wondering if we have to add it to

Nope, before, not all pages were marked reserved. See the patch description.

> node 0 and zone 0. From below commit, I don't get why. Could you help
> clarify so that I get what I missed?

Node 0 / zone 0 is just like zeroing it IIRC - no change in that regard,
my patch just called that out explicitly.

-- 
Thanks,

David / dhildenb