linux-kernel - Re: [PATCH v2 2/3] mm/swapfile: Fix lost swap bits in unuse

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Message-ID: <67fc9368-0876-b931-14c2-ffa4dac35b6d@redhat.com>
Date:   Thu, 21 Apr 2022 15:13:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, apopple@...dia.com, surenb@...gle.com,
        minchan@...nel.org, peterx@...hat.com, sfr@...b.auug.org.au,
        naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mm/swapfile: Fix lost swap bits in unuse_pte()

On 21.04.22 14:53, Miaohe Lin wrote:
> This is observed by code review only but not any real report.
> 
> When we turn off swapping we could have lost the bits stored in the swap
> ptes. The new rmap-exclusive bit is fine since that turned into a page
> flag, but not for soft-dirty and uffd-wp. Add them.
> 
> Suggested-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/swapfile.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 95b63f69f388..332ccfc76142 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1783,7 +1783,7 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  {
>  	struct page *swapcache;
>  	spinlock_t *ptl;
> -	pte_t *pte;
> +	pte_t *pte, new_pte;
>  	int ret = 1;
>  
>  	swapcache = page;
> @@ -1832,8 +1832,14 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  		page_add_new_anon_rmap(page, vma, addr);
>  		lru_cache_add_inactive_or_unevictable(page, vma);
>  	}
> -	set_pte_at(vma->vm_mm, addr, pte,
> -		   pte_mkold(mk_pte(page, vma->vm_page_prot)));
> +	new_pte = pte_mkold(mk_pte(page, vma->vm_page_prot));
> +	if (pte_swp_soft_dirty(*pte))
> +		new_pte = pte_mksoft_dirty(new_pte);
> +	if (pte_swp_uffd_wp(*pte)) {
> +		new_pte = pte_mkuffd_wp(new_pte);
> +		new_pte = pte_wrprotect(new_pte);

The wrprotect shouldn't be necessary, we don't do a pte_mkwrite(). Note
that in do_swap_page() we might have done a
maybe_mkwrite(pte_mkdirty(pte)), which is why the pte_wrprotect() is
required there.

-- 
Thanks,

David / dhildenb