linux-kernel - Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap read fails

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Message-ID: <Yl8rZkhU/B0iE2ob@xz-m1.local>
Date:   Tue, 19 Apr 2022 17:36:38 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, vbabka@...e.cz,
        dhowells@...hat.com, neilb@...e.de, david@...hat.com,
        apopple@...dia.com, surenb@...gle.com, minchan@...nel.org,
        sfr@...b.auug.org.au, rcampbell@...dia.com,
        naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap
 read fails

On Sat, Apr 16, 2022 at 11:05:49AM +0800, Miaohe Lin wrote:
> @@ -1797,6 +1797,17 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  		goto out;
>  	}
>  
> +	if (unlikely(!PageUptodate(page))) {
> +		pte_t pteval;
> +
> +		dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> +		pteval = swp_entry_to_pte(make_swapin_error_entry(page));
> +		set_pte_at(vma->vm_mm, addr, pte, pteval);
> +		swap_free(entry);
> +		ret = 0;
> +		goto out;
> +	}
> +
>  	/* See do_swap_page() */
>  	BUG_ON(!PageAnon(page) && PageMappedToDisk(page));
>  	BUG_ON(PageAnon(page) && PageAnonExclusive(page));

Totally off-topic, but.. today when I was looking at the unuse path I just
found that the swp bits could have got lost for either soft-dirty and
uffd-wp here?  A quick patch attached.

Maybe at some point we should start to have some special helpers for
set_pte_at() when we're converting between present/non-present ptes, so as
to make sure all these will always be taken care of properly.

-- 
Peter Xu

View attachment "0001-mm-swap-Fix-lost-swap-bits-in-unuse_pte.patch" of type "text/plain" (1522 bytes)