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

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

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

I'm preparing posting the v2. But I found this won't fix the issue if CONFIG_KSM is disabled. So the correct fix
should be something like below?

diff --git a/mm/ksm.c b/mm/ksm.c
index 97a9627116fa..74804158ee02 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2794,6 +2794,8 @@ struct page *ksm_might_need_to_copy(struct page *page,
                        anon_vma->root == vma->anon_vma->root) {
                return page;            /* still no need to copy it */
        }
+       if (PageHWPoison(page))
+               return ERR_PTR(-EHWPOISON);
        if (!PageUptodate(page))
                return page;            /* let do_swap_page report the error */

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 346e22b8ae97..3dcc9d92689c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1744,7 +1744,7 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
        struct page *swapcache;
        spinlock_t *ptl;
        pte_t *pte, new_pte, old_pte;
-       bool hwposioned = false;
+       bool hwposioned = PageHWPoison(page);
        int ret = 1;

        swapcache = page;


Thanks.