Re: [PATCH 2/4] mm: memory_hotplug: check hwpoisoned page firstly in do_migrate_range()

[David Hildenbrand <david@redhat.com>]

On 25.07.24 03:16, Kefeng Wang wrote:
> The commit b15c87263a69 ("hwpoison, memory_hotplug: allow hwpoisoned
> pages to be offlined") don't handle the hugetlb pages, the dead loop
> still occur if offline a hwpoison hugetlb, luckly, after the commit
> e591ef7d96d6 ("mm,hwpoison,hugetlb,memory_hotplug: hotremove memory
> section with hwpoisoned hugepage"), the HPageMigratable of hugetlb
> page will be clear, and the hwpoison hugetlb page will be skipped in
> scan_movable_pages(), so the deed loop issue is fixed.
> 
> However if the HPageMigratable() check passed(without reference and
> lock), the hugetlb page may be hwpoisoned, it won't cause issue since
> the hwpoisoned page will be handled correctly in the next movable
> pages scan loop, and it will be isolated in do_migrate_range() and
> but fails to migrated. In order to avoid the unnecessary isolation and
> unify all hwpoisoned page handling, let's unconditionally check hwpoison
> firstly, and if it is a hwpoisoned hugetlb page, try to unmap it as
> the catch all safety net like normal page does.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>   mm/memory_hotplug.c | 27 ++++++++++++++++-----------
>   1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 66267c26ca1b..ccaf4c480aed 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1788,28 +1788,33 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>   		folio = page_folio(page);
>   		head = &folio->page;
>   
> -		if (PageHuge(page)) {
> -			pfn = page_to_pfn(head) + compound_nr(head) - 1;
> -			isolate_hugetlb(folio, &source);
> -			continue;
> -		} else if (PageTransHuge(page))
> -			pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;
> -
>   		/*
>   		 * HWPoison pages have elevated reference counts so the migration would
>   		 * fail on them. It also doesn't make any sense to migrate them in the
>   		 * first place. Still try to unmap such a page in case it is still mapped
> -		 * (e.g. current hwpoison implementation doesn't unmap KSM pages but keep
> -		 * the unmap as the catch all safety net).
> +		 * (keep the unmap as the catch all safety net).
>   		 */
> -		if (PageHWPoison(page)) {
> +		if (unlikely(PageHWPoison(page))) {
> +			folio = page_folio(page);
> +
>   			if (WARN_ON(folio_test_lru(folio)))
>   				folio_isolate_lru(folio);
> +
>   			if (folio_mapped(folio))
> -				try_to_unmap(folio, TTU_IGNORE_MLOCK);
> +				unmap_posioned_folio(folio, TTU_IGNORE_MLOCK);
> +
> +			if (folio_test_large(folio))
> +				pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
>   			continue;
>   		}
>   
> +		if (PageHuge(page)) {
> +			pfn = page_to_pfn(head) + compound_nr(head) - 1;
> +			isolate_hugetlb(folio, &source);
> +			continue;
> +		} else if (PageTransHuge(page))
> +			pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;

If we can use a folio in the PageHWPoison() case, can we use one here as 
well? I know that it's all unreliable when not holding a folio 
reference, and we have to be a bit careful.

It feels like using folios here would mostly be fine, because things 
like PageHuge() already use folios internally.

And using it in the PageHWPoison() but not here looks a bit odd.

The important part is that we don't segfault if we'd overshoot our target.

-- 
Cheers,

David / dhildenb