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

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Date:   Mon, 17 Jul 2023 03:53:55 +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 Mon, Jul 17, 2023 at 10:33:14AM +0800, Miaohe Lin wrote:
> On 2023/7/15 11:50, Matthew Wilcox wrote:
> > 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?
> 
> hwposioned will be true iff ksm_might_need_to_copy returns -EHWPOISON.
> And there's PageUptodate check in ksm_might_need_to_copy before we can return -EHWPOISON:
> 
>   ksm_might_need_to_copy
>     if (!PageUptodate(page))
>       return page;		/* let do_swap_page report the error */
>     ^^^
>     Will return here because hwpoisoned swapcache page is !PageUptodate(cleared via me_swapcache_dirty()).
> 
> Or am I miss something?

Ah!  So we don't even get to calling copy_mc_to_kernel().  That seems
like a bug in ksm_might_need_to_copy(), don't you think?  Maybe this
would be a better fix:

+	if (PageHWPoison(page))
+		return ERR_PTR(-EHWPOISON);
	if (!PageUptodate(page))
		return page;