Re: [PATCH 1/2] mm/memory_hotplug: remove head page reference in do_migrate_range

[David Hildenbrand <david@redhat.com>]

On 23.01.23 21:37, Matthew Wilcox wrote:
> On Mon, Jan 23, 2023 at 12:23:46PM -0800, Sidhartha Kumar wrote:
>> @@ -1637,14 +1637,13 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>>   			continue;
>>   		page = pfn_to_page(pfn);
>>   		folio = page_folio(page);
>> -		head = &folio->page;
>>   
>> -		if (PageHuge(page)) {
>> -			pfn = page_to_pfn(head) + compound_nr(head) - 1;
>> +		if (folio_test_hugetlb(folio)) {
>> +			pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
>>   			isolate_hugetlb(folio, &source);
>>   			continue;
>> -		} else if (PageTransHuge(page))
>> -			pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;
>> +		} else if (folio_test_transhuge(folio))
>> +			pfn = folio_pfn(folio) + thp_nr_pages(page) - 1;
> 
> I'm pretty sure those two lines should be...
> 
> 		} else if (folio_test_large(folio))
> 			pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
> 
> But, erm ... we're doing this before we have a refcount on the page,
> right?  So this is unsafe because the page might change which folio
> it is in.  And the folio we found earlier might become a tail page
> of a different folio.  (As the comment below explains, HWPoison pages
> won't, so it's not unsafe for them).
> 
> Also, thp_nr_pages(page) is going to return 1 for tail pages.  So this
> is a noop, unless page is a head page.
> 
> It's all a bit confusing, and being memory-hotplug, it's not well
> tested.  More thought needed.

Ehm, it is fairly well tested ;)

As memory offlining keeps retrying, temporarily making wrong assumptions 
about a folio is acceptable, as long as we don't run into BUGs.

It's certainly worth a big comment in a code, that this is all racy and 
that page migration code will stabilize.

Now, we could temporarily take a reference, but ... common migration 
code will try taking its own ref to stabilize the page and would be 
confused about yet another ref (-> migration will fail).

So we have to be careful about grabbing references on these pages, and 
how long we're going to hold them. Otherwise we'll break memory 
offlining completely :)

-- 
Thanks,

David / dhildenb