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 10:33:14 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
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 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?

Thanks.