linux-kernel - Re: [PATCH 1/4] mm/swapfile: fix wrong swap entry type for hwpoisoned swapcache page

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Date:   Sat, 15 Jul 2023 04:50:27 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, naoya.horiguchi@....com,
        shy828301@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] mm/swapfile: fix wrong swap entry type for
 hwpoisoned swapcache page

On Sat, Jul 15, 2023 at 11:17:26AM +0800, Miaohe Lin wrote:
> Hwpoisoned dirty swap cache page is kept in the swap cache and there's
> simple interception code in do_swap_page() to catch it. But when trying
> to swapoff, unuse_pte() will wrongly install a general sense of "future
> accesses are invalid" swap entry for hwpoisoned swap cache page due to
> unaware of such type of page. The user will receive SIGBUS signal without
> expected BUS_MCEERR_AR payload.

Have you observed this, or do you just think it's true?

> +++ b/mm/swapfile.c
> @@ -1767,7 +1767,8 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  		swp_entry_t swp_entry;
>  
>  		dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> -		if (hwposioned) {
> +		/* Hwpoisoned swapcache page is also !PageUptodate. */
> +		if (hwposioned || PageHWPoison(page)) {

This line makes no sense to me.  How do we get here with PageHWPoison()
being true and hwposioned being false?

>  			swp_entry = make_hwpoison_entry(swapcache);
>  			page = swapcache;
>  		} else {
> -- 
> 2.33.0
> 
>