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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2023/7/17 10:53, Matthew Wilcox wrote:
> 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

I'm sorry but could you please explain what the bug is?

> would be a better fix:
> 
> +	if (PageHWPoison(page))
> +		return ERR_PTR(-EHWPOISON);

Looks reasonable. We can further avoid accessing contents of hwpoisoned swapcache pages which are
PageHWPoison() && PageUptodate() at first place.

Thanks.

> 	if (!PageUptodate(page))
> 		return page;
> 
> .
>