linux-kernel - Re: [PATCH 1/2] mm/memory_hotplug: remove head page reference in do_migrate

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Date:   Mon, 23 Jan 2023 13:08:49 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, david@...hat.com, osalvador@...e.de
Subject: Re: [PATCH 1/2] mm/memory_hotplug: remove head page reference in
 do_migrate_range

On 1/23/23 12:37 PM, 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).
> 

Thanks for the explanation of why this is unsafe. Would it be worth to 
put this code block inside the

		if (!get_page_unless_zero(page))
			continue;

		put_page(page);

block found lower? My motivation for this series is the HPageMigratable 
call in patch 2 is the last user of the huge page flag test macros so a 
conversion would allow for the removal of the macro. I thought I could 
also remove the head page references found in this function, but if that 
would cause too much churn in a complicated sub-system it can be dropped.

Thanks,
Sidhartha Kumar
> 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.