Re: [mm-unstable PATCH v6 3/8] mm, hwpoison, hugetlb: support saving mechanism of raw error pages

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/12 11:28, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.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@nec.com>
> Reported-by: kernel test robot <lkp@intel.com>

This patch looks good to me with some nits below.

> ---
...
> +static 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 = TestSetPageHWPoison(hpage) ? -EHWPOISON : 0;
> +
> +	/*
> +	 * Once the hwpoison hugepage has lost reliable raw error info,
> +	 * there is little meaning to keep additional error info precisely,
> +	 * so skip to add additional raw error info.
> +	 */
> +	if (HPageRawHwpUnreliable(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;
> +	}
> +
> +	raw_hwp = kmalloc(sizeof(struct raw_hwp_page), GFP_ATOMIC);
> +	if (raw_hwp) {
> +		raw_hwp->page = page;
> +		llist_add(&raw_hwp->node, head);
> +		/* the first error event will be counted in action_result(). */
> +		if (ret)
> +			num_poisoned_pages_inc();
> +	} else {
> +		/*
> +		 * Failed to save raw error info.  We no longer trace all
> +		 * hwpoisoned subpages, and we need refuse to free/dissolve
> +		 * this hwpoisoned hugepage.
> +		 */
> +		SetHPageRawHwpUnreliable(hpage);

IMHO, when HPageRawHwpUnreliable is set, we can simply free the raw_hwp_list here
to save some memory as they're not used anymore.

> +	}
> +	return ret;
> +}
> +
> +void hugetlb_clear_page_hwpoison(struct page *hpage)
> +{
> +	struct llist_head *head;
> +	struct llist_node *t, *tnode;
> +
> +	if (!HPageRawHwpUnreliable(hpage))
> +		ClearPageHWPoison(hpage);
> +	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);
> +
> +		SetPageHWPoison(p->page);
> +		kfree(p);
> +	}
> +	llist_del_all(head);
> +}
> +
>  /*
>   * Called from hugetlb code with hugetlb_lock held.
>   *
> @@ -1698,7 +1771,7 @@ 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;
>  	}
> @@ -1710,7 +1783,6 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
>  	return ret;
>  }
>  
> -#ifdef CONFIG_HUGETLB_PAGE
>  /*
>   * Taking refcount of hugetlb pages needs extra care about race conditions
>   * with basic operations like hugepage allocation/free/demotion.
> @@ -1751,7 +1823,7 @@ 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);

If the code reach here, the hugetlb page should be poisoned first time or it will return
in above "res == -EHWPOISON" case. So ClearPageHWPoison should be fine here. But this change
will do the same thing while making the code more consistent. So it should be fine. :)

>  		res = -EOPNOTSUPP;
>  		goto out;
>  	}
> 

Anyway, for what it worth,

Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

Thanks!