Re: [RFC PATCH v1 2/4] mm,hwpoison,hugetlb,memory_hotplug: hotremove memory section with hwpoisoned hugepage

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/4/27 12:28, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> HWPoisoned page is not supposed to prevent memory hotremove, but
> currently this does not properly work for hwpoisoned hugepages and the
> kernel tries to migrate them, which could cause consuming corrupted
> data.
> 
> Move dissolve_free_huge_pages() before scan_movable_pages(). This is
> because the result of the movable check depends on the result of the
> dissolve.  Now delayed dissolve is available, so hwpoisoned hugepages
> can be turned into 4kB hwpoison page which memory hotplug can handle.
> 
> And clear HPageMigratable pseudo flag for hwpoisoned hugepages. This is
> also important because dissolve_free_huge_page() can fail.  So it's
> still necessary to prevent do_migrate_pages() from trying to migrate
> hwpoison hugepages.
> 
> Reported-by: Miaohe Lin <linmiaohe@huawei.com>
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> ---
>  mm/hugetlb.c        | 11 +++++++++++
>  mm/memory-failure.c |  2 ++
>  mm/memory_hotplug.c | 23 +++++++++++------------
>  3 files changed, 24 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6867ea8345d1..95b1db852ca9 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2159,6 +2159,17 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
>  
>  	for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
>  		page = pfn_to_page(pfn);
> +
> +		if (PageHuge(page) && PageHWPoison(page)) {
> +			/*
> +			 * Release the last refcount from hwpoison to turn into
> +			 * a free hugepage.
> +			 */
> +			if (page_count(page) == 1)
> +				put_page(page);
> +			page = hugetlb_page_hwpoison(page);
> +		}
> +

This patch looks good to me. Thanks!

One question: Can this hugepage be put into buddy system? In free_huge_page,
if h->surplus_huge_pages_node[nid] > 0, hugepage might be put into buddy via
update_and_free_page. So it's not PageHuge anymore and won't be dissolved. If
this happens, the "raw error page" is still missed and might be accessed later.
Or am I miss something?

Thanks!

>  		rc = dissolve_free_huge_page(page);
>  		if (rc)
>  			break;
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 73948a00ad4a..4a2e22bf0983 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1607,6 +1607,8 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
>  		return res == MF_RECOVERED ? 0 : -EBUSY;
>  	}
>  
> +	ClearHPageMigratable(head);
> +
>  	page_flags = head->flags;
>  
>  	/*
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 416b38ca8def..4bc0590f4334 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1864,6 +1864,17 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  
>  			cond_resched();
>  
> +			/*
> +			 * Dissolve free hugepages in the memory block before doing
> +			 * offlining actually in order to make hugetlbfs's object
> +			 * counting consistent.
> +			 */
> +			ret = dissolve_free_huge_pages(start_pfn, end_pfn);
> +			if (ret) {
> +				reason = "failure to dissolve huge pages";
> +				goto failed_removal_isolated;
> +			}
> +
>  			ret = scan_movable_pages(pfn, end_pfn, &pfn);
>  			if (!ret) {
>  				/*
> @@ -1879,19 +1890,7 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  			goto failed_removal_isolated;
>  		}
>  
> -		/*
> -		 * Dissolve free hugepages in the memory block before doing
> -		 * offlining actually in order to make hugetlbfs's object
> -		 * counting consistent.
> -		 */
> -		ret = dissolve_free_huge_pages(start_pfn, end_pfn);
> -		if (ret) {
> -			reason = "failure to dissolve huge pages";
> -			goto failed_removal_isolated;
> -		}
> -
>  		ret = test_pages_isolated(start_pfn, end_pfn, MEMORY_OFFLINE);
> -
>  	} while (ret);
>  
>  	/* Mark all sections offline and remove free pages from the buddy. */
>