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Message-ID: <5c40ed66-1e51-78fa-193c-0eb0db814b01@oracle.com>
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.
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