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 20:37:59 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Sidhartha Kumar <sidhartha.kumar@...cle.com>
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 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.