Re: [RFC PATCH v1 1/4] mm, hwpoison, hugetlb: introduce SUBPAGE_INDEX_HWPOISON to save raw error page

[Jane Chu]

On 4/26/2022 9:28 PM, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> 
> When handling memory error on a hugetlb page, the error handler tries to
> dissolve and turn it into 4kB pages.  If it's successfully dissolved,
> PageHWPoison flag is moved to the raw error page, so but that's all
> right.  However, dissolve sometimes fails, then the error page is left
> as hwpoisoned hugepage. It's useful if we can retry to dissolve it to
> save healthy pages, but that's not possible now because the information
> about where the raw error page is lost.
> 
> Use the private field of a tail page to keep that information.  The code
> path of shrinking hugepage pool used this info to try delayed dissolve.
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> ---
>   include/linux/hugetlb.h | 24 ++++++++++++++++++++++++
>   mm/hugetlb.c            |  9 +++++++++
>   mm/memory-failure.c     |  2 ++
>   3 files changed, 35 insertions(+)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index ac2a1d758a80..689e69cb556b 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -42,6 +42,9 @@ enum {
>   	SUBPAGE_INDEX_CGROUP,		/* reuse page->private */
>   	SUBPAGE_INDEX_CGROUP_RSVD,	/* reuse page->private */
>   	__MAX_CGROUP_SUBPAGE_INDEX = SUBPAGE_INDEX_CGROUP_RSVD,
> +#endif
> +#ifdef CONFIG_CGROUP_HUGETLB
> +	SUBPAGE_INDEX_HWPOISON,
>   #endif
>   	__NR_USED_SUBPAGE,
>   };
> @@ -784,6 +787,27 @@ extern int dissolve_free_huge_page(struct page *page);
>   extern int dissolve_free_huge_pages(unsigned long start_pfn,
>   				    unsigned long end_pfn);
>   
> +#ifdef CONFIG_MEMORY_FAILURE
> +/*
> + * pointer to raw error page is located in hpage[SUBPAGE_INDEX_HWPOISON].private
> + */
> +static inline struct page *hugetlb_page_hwpoison(struct page *hpage)
> +{
> +	return (void *)page_private(hpage + SUBPAGE_INDEX_HWPOISON);
> +}
> +
> +static inline void hugetlb_set_page_hwpoison(struct page *hpage,
> +					struct page *page)
> +{
> +	set_page_private(hpage + SUBPAGE_INDEX_HWPOISON, (unsigned long)page);
> +}

What happens if the ->private field is already holding a poisoned page 
pointer?  that is, in a scenario of multiple poisoned pages within a 
hugepage, what to do?  mark the entire hpage poisoned?

thanks,
-jane


> +#else
> +static inline struct page *hugetlb_page_hwpoison(struct page *hpage)
> +{
> +	return NULL;
> +}
> +#endif
> +
>   #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
>   #ifndef arch_hugetlb_migration_supported
>   static inline bool arch_hugetlb_migration_supported(struct hstate *h)
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f8e048b939c7..6867ea8345d1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1547,6 +1547,15 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
>   		return;
>   	}
>   
> +	if (unlikely(PageHWPoison(page))) {
> +		struct page *raw_error = hugetlb_page_hwpoison(page);
> +
> +		if (raw_error && raw_error != page) {
> +			SetPageHWPoison(raw_error);
> +			ClearPageHWPoison(page);
> +		}
> +	}
> +
>   	for (i = 0; i < pages_per_huge_page(h);
>   	     i++, subpage = mem_map_next(subpage, page, i)) {
>   		subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 3e36fc19c4d1..73948a00ad4a 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1535,6 +1535,8 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
>   		goto out;
>   	}
>   
> +	hugetlb_set_page_hwpoison(head, page);
> +
>   	return ret;
>   out:
>   	if (count_increased)