Date:   Mon, 27 Jun 2022 17:26:01 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Naoya Horiguchi <nao.horiguchi@...il.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/9] mm, hwpoison, hugetlb: support saving mechanism
 of raw error pages

On Fri, Jun 24, 2022 at 08:51:48AM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@....com>
> 
> 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 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 pages is lost.
> 
> Use the private field of a few tail pages to keep that information.  The
> code path of shrinking hugepage pool uses this info to try delayed dissolve.
> In order to remember multiple errors in a hugepage, a singly-linked list
> originated from SUBPAGE_INDEX_HWPOISON-th tail page is constructed.  Only
> simple operations (adding an entry or clearing all) are required and the
> list is assumed not to be very long, so this simple data structure should
> be enough.
> 
> If we failed to save raw error info, the hwpoison hugepage has errors on
> unknown subpage, then this new saving mechanism does not work any more,
> so disable saving new raw error info and freeing hwpoison hugepages.
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>

Thanks for your work on this. I have several quesions below.

> ---
> v1 -> v2:
> - support hwpoison hugepage with multiple errors,
> - moved the new interface functions to mm/memory-failure.c,
> - define additional subpage index SUBPAGE_INDEX_HWPOISON_UNRELIABLE,
> - stop freeing/dissolving hwpoison hugepages with unreliable raw error info,
> - drop hugetlb_clear_page_hwpoison() in dissolve_free_huge_page() because
>   that's done in update_and_free_page(),
> - move setting/clearing PG_hwpoison flag to the new interfaces,
> - checking already hwpoisoned or not on a subpage basis.
> 
> ChangeLog since previous post on 4/27:
> - fixed typo in patch description (by Miaohe)
> - fixed config value in #ifdef statement (by Miaohe)
> - added sentences about "multiple hwpoison pages" scenario in patch
>   description
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
> ---
>  include/linux/hugetlb.h | 13 ++++++
>  mm/hugetlb.c            | 39 +++++++++--------
>  mm/memory-failure.c     | 95 ++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 125 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index e4cff27d1198..ac13c2022517 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -42,6 +42,10 @@ 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_MEMORY_FAILURE
> +	SUBPAGE_INDEX_HWPOISON,
> +	SUBPAGE_INDEX_HWPOISON_UNRELIABLE,
>  #endif
>  	__NR_USED_SUBPAGE,
>  };
> @@ -798,6 +802,15 @@ 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
> +extern int hugetlb_clear_page_hwpoison(struct page *hpage);
> +#else
> +static inline int hugetlb_clear_page_hwpoison(struct page *hpage)
> +{
> +	return 0;
> +}
> +#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 b7ae5f73f3b2..19ef427aa1e8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1541,17 +1541,15 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
>  	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
>  		return;
>  
> -	if (hugetlb_vmemmap_alloc(h, page)) {
> -		spin_lock_irq(&hugetlb_lock);
> -		/*
> -		 * If we cannot allocate vmemmap pages, just refuse to free the
> -		 * page and put the page back on the hugetlb free list and treat
> -		 * as a surplus page.
> -		 */
> -		add_hugetlb_page(h, page, true);
> -		spin_unlock_irq(&hugetlb_lock);
> -		return;
> -	}
> +	if (hugetlb_vmemmap_alloc(h, page))
> +		goto fail;
> +
> +	/*
> +	 * Move PageHWPoison flag from head page to the raw error pages,
> +	 * which makes any healthy subpages reusable.
> +	 */
> +	if (unlikely(PageHWPoison(page) && hugetlb_clear_page_hwpoison(page)))
> +		goto fail;
>  
>  	for (i = 0; i < pages_per_huge_page(h);
>  	     i++, subpage = mem_map_next(subpage, page, i)) {
> @@ -1572,6 +1570,16 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
>  	} else {
>  		__free_pages(page, huge_page_order(h));
>  	}
> +	return;
> +fail:
> +	spin_lock_irq(&hugetlb_lock);
> +	/*
> +	 * If we cannot allocate vmemmap pages or cannot identify raw hwpoison
> +	 * subpages reliably, just refuse to free the page and put the page
> +	 * back on the hugetlb free list and treat as a surplus page.
> +	 */
> +	add_hugetlb_page(h, page, true);
> +	spin_unlock_irq(&hugetlb_lock);
>  }
>  
>  /*
> @@ -2115,15 +2123,6 @@ int dissolve_free_huge_page(struct page *page)
>  		 */
>  		rc = hugetlb_vmemmap_alloc(h, head);
>  		if (!rc) {
> -			/*
> -			 * Move PageHWPoison flag from head page to the raw
> -			 * error page, which makes any subpages rather than
> -			 * the error page reusable.
> -			 */
> -			if (PageHWPoison(head) && page != head) {
> -				SetPageHWPoison(page);
> -				ClearPageHWPoison(head);
> -			}
>  			update_and_free_page(h, head, false);
>  		} else {
>  			spin_lock_irq(&hugetlb_lock);
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index fb3feb1f363e..af571fa6f2af 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1499,6 +1499,97 @@ static int try_to_split_thp_page(struct page *page, const char *msg)
>  }
>  
>  #ifdef CONFIG_HUGETLB_PAGE
> +/*
> + * Struct raw_hwp_page represents information about "raw error page",
> + * constructing singly linked list originated from ->private field of
> + * SUBPAGE_INDEX_HWPOISON-th tail page.
> + */
> +struct raw_hwp_page {
> +	struct llist_node node;
> +	struct page *page;
> +};
> +
> +static inline struct llist_head *raw_hwp_list_head(struct page *hpage)
> +{
> +	return (struct llist_head *)&page_private(hpage + SUBPAGE_INDEX_HWPOISON);
> +}
> +
> +static inline int raw_hwp_unreliable(struct page *hpage)
> +{
> +	return page_private(hpage + SUBPAGE_INDEX_HWPOISON_UNRELIABLE);
> +}
> +
> +static inline void set_raw_hwp_unreliable(struct page *hpage)
> +{
> +	set_page_private(hpage + SUBPAGE_INDEX_HWPOISON_UNRELIABLE, 1);
> +}

Why not use HPAGEFLAG(HwpoisonUnreliable, hwpoison_unreliable) directly?

> +
> +/*
> + * about race consideration
> + */
> +static inline int hugetlb_set_page_hwpoison(struct page *hpage,
> +					struct page *page)
> +{
> +	struct llist_head *head;
> +	struct raw_hwp_page *raw_hwp;
> +	struct llist_node *t, *tnode;
> +	int ret;
> +
> +	/*
> +	 * Once the hwpoison hugepage has lost reliable raw error info,
> +	 * there is little mean to keep additional error info precisely,
> +	 * so skip to add additional raw error info.
> +	 */
> +	if (raw_hwp_unreliable(hpage))
> +		return -EHWPOISON;
> +	head = raw_hwp_list_head(hpage);
> +	llist_for_each_safe(tnode, t, head->first) {
> +		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
> +
> +		if (p->page == page)
> +			return -EHWPOISON;
> +	}
> +
> +	ret = TestSetPageHWPoison(hpage) ? -EHWPOISON : 0;
> +	/* the first error event will be counted in action_result(). */
> +	if (ret)
> +		num_poisoned_pages_inc();
> +
> +	raw_hwp = kmalloc(sizeof(struct raw_hwp_page), GFP_KERNEL);

This function can be called in atomic context, GFP_ATOMIC should be used
here.

> +	if (raw_hwp) {
> +		raw_hwp->page = page;
> +		llist_add(&raw_hwp->node, head);

The maximum amount of items in the list is one, right?

> +	} else {
> +		/*
> +		 * Failed to save raw error info.  We no longer trace all
> +		 * hwpoisoned subpages, and we need refuse to free/dissolve
> +		 * this hwpoisoned hugepage.
> +		 */
> +		set_raw_hwp_unreliable(hpage);
> +		return ret;
> +	}
> +	return ret;
> +}
> +
> +inline int hugetlb_clear_page_hwpoison(struct page *hpage)
> +{
> +	struct llist_head *head;
> +	struct llist_node *t, *tnode;
> +
> +	if (raw_hwp_unreliable(hpage))
> +		return -EBUSY;

IIUC, we use head page's PageHWPoison to synchronize hugetlb_clear_page_hwpoison()
and hugetlb_set_page_hwpoison(), right? If so, who can set hwp_unreliable here?

> +	ClearPageHWPoison(hpage);
> +	head = raw_hwp_list_head(hpage);
> +	llist_for_each_safe(tnode, t, head->first) {

Is it possible that a new item is added hugetlb_set_page_hwpoison()  and we do not
traverse it (we have cleared page's PageHWPoison)? Then we ignored a real hwpoison
page, right?

> +		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
> +
> +		SetPageHWPoison(p->page);
> +		kfree(p);
> +	}
> +	llist_del_all(head);

If the above issue exists, moving ClearPageHWPoison(hpage) to here could
fix it. We should clear PageHWPoison carefully since the head page is
also can be poisoned.

Thanks.

> +	return 0;
> +}
> +
>  /*
>   * Called from hugetlb code with hugetlb_lock held.
>   *
> @@ -1533,7 +1624,7 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
>  		goto out;
>  	}
>  
> -	if (TestSetPageHWPoison(head)) {
> +	if (hugetlb_set_page_hwpoison(head, page)) {
>  		ret = -EHWPOISON;
>  		goto out;
>  	}
> @@ -1585,7 +1676,7 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
>  	lock_page(head);
>  
>  	if (hwpoison_filter(p)) {
> -		ClearPageHWPoison(head);
> +		hugetlb_clear_page_hwpoison(head);
>  		res = -EOPNOTSUPP;
>  		goto out;
>  	}
> -- 
> 2.25.1
> 
> 

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