linux-kernel - Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap read fails

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Message-ID: <8aeebc2f0b2a251d3d70402cd0edf063ba911013.camel@intel.com>
Date:   Mon, 25 Apr 2022 15:41:25 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, david@...hat.com, 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 v3 1/3] mm/swapfile: unuse_pte can map random data if
 swap read fails

Hi, Miaohe,

On Sun, 2022-04-24 at 17:11 +0800, Miaohe Lin wrote:
> There is a bug in unuse_pte(): when swap page happens to be unreadable,
> page filled with random data is mapped into user address space.  In case
> of error, a special swap entry indicating swap read fails is set to the
> page table.  So the swapcache page can be freed and the user won't end up
> with a permanently mounted swap because a sector is bad.  And if the page
> is accessed later, the user process will be killed so that corrupted data
> is never consumed.  On the other hand, if the page is never accessed, the
> user won't even notice it.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> Acked-by: David Hildenbrand <david@...hat.com>
> ---
>  include/linux/swap.h    |  7 ++++++-
>  include/linux/swapops.h | 10 ++++++++++
>  mm/memory.c             |  5 ++++-
>  mm/swapfile.c           | 11 +++++++++++
>  4 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 5553189d0215..b82c196d8867 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -55,6 +55,10 @@ static inline int current_is_kswapd(void)
>   * actions on faults.
>   */
> 
> +#define SWP_SWAPIN_ERROR_NUM 1
> +#define SWP_SWAPIN_ERROR     (MAX_SWAPFILES + SWP_HWPOISON_NUM + \
> +			     SWP_MIGRATION_NUM + SWP_DEVICE_NUM + \
> +			     SWP_PTE_MARKER_NUM)
> 
> 

It appears wasteful to use another swap device number.  Is it possible
to use a special swap offset?  For example, 0 or -1?

Best Regards,
Huang, Ying


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