linux-kernel - Re: [PATCH 4/4] mm: hwpoison: handle non-anonymous THP correctly

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Date:   Tue, 21 Sep 2021 18:50:34 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     Yang Shi <shy828301@...il.com>
Cc:     naoya.horiguchi@....com, hughd@...gle.com,
        kirill.shutemov@...ux.intel.com, willy@...radead.org,
        osalvador@...e.de, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] mm: hwpoison: handle non-anonymous THP correctly

On Tue, Sep 14, 2021 at 11:37:18AM -0700, Yang Shi wrote:
> Currently hwpoison doesn't handle non-anonymous THP, but since v4.8 THP
> support for tmpfs and read-only file cache has been added.  They could
> be offlined by split THP, just like anonymous THP.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/memory-failure.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 3e06cb9d5121..6f72aab8ec4a 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1150,13 +1150,16 @@ static int __get_hwpoison_page(struct page *page)
>  
>  	if (PageTransHuge(head)) {
>  		/*
> -		 * Non anonymous thp exists only in allocation/free time. We
> -		 * can't handle such a case correctly, so let's give it up.
> -		 * This should be better than triggering BUG_ON when kernel
> -		 * tries to touch the "partially handled" page.
> +		 * We can't handle allocating or freeing THPs, so let's give
> +		 * it up. This should be better than triggering BUG_ON when
> +		 * kernel tries to touch the "partially handled" page.
> +		 *
> +		 * page->mapping won't be initialized until the page is added
> +		 * to rmap or page cache.  Use this as an indicator for if
> +		 * this is an instantiated page.
>  		 */
> -		if (!PageAnon(head)) {
> -			pr_err("Memory failure: %#lx: non anonymous thp\n",
> +		if (!head->mapping) {
> +			pr_err("Memory failure: %#lx: non instantiated thp\n",
>  				page_to_pfn(page));
>  			return 0;
>  		}

How about cleaning up this whole "PageTransHuge()" block?  As explained in
commit 415c64c1453a (mm/memory-failure: split thp earlier in memory error
handling), this check was introduced to avoid that non-anonymous thp is
considered as hugetlb and code for hugetlb is executed (resulting in crash).

With recent improvement in __get_hwpoison_page(), this confusion never
happens (because hugetlb check is done before this check), so this check
seems to finish its role.

Thanks,
Naoya Horiguchi

> @@ -1415,12 +1418,12 @@ static int identify_page_state(unsigned long pfn, struct page *p,
>  static int try_to_split_thp_page(struct page *page, const char *msg)
>  {
>  	lock_page(page);
> -	if (!PageAnon(page) || unlikely(split_huge_page(page))) {
> +	if (!page->mapping || unlikely(split_huge_page(page))) {
>  		unsigned long pfn = page_to_pfn(page);
>  
>  		unlock_page(page);
> -		if (!PageAnon(page))
> -			pr_info("%s: %#lx: non anonymous thp\n", msg, pfn);
> +		if (!page->mapping)
> +			pr_info("%s: %#lx: not instantiated thp\n", msg, pfn);
>  		else
>  			pr_info("%s: %#lx: thp split failed\n", msg, pfn);
>  		put_page(page);
> -- 
> 2.26.2
>