linux-kernel - Re: [PATCH v2 2/3] mm/swapfile: Fix lost swap bits in unuse

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Date:   Thu, 21 Apr 2022 21:50:38 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>, <akpm@...ux-foundation.org>
CC:     <willy@...radead.org>, <vbabka@...e.cz>, <dhowells@...hat.com>,
        <neilb@...e.de>, <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 v2 2/3] mm/swapfile: Fix lost swap bits in unuse_pte()

On 2022/4/21 21:13, David Hildenbrand wrote:
> On 21.04.22 14:53, Miaohe Lin wrote:
>> This is observed by code review only but not any real report.
>>
>> When we turn off swapping we could have lost the bits stored in the swap
>> ptes. The new rmap-exclusive bit is fine since that turned into a page
>> flag, but not for soft-dirty and uffd-wp. Add them.
>>
>> Suggested-by: Peter Xu <peterx@...hat.com>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/swapfile.c | 12 +++++++++---
>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 95b63f69f388..332ccfc76142 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1783,7 +1783,7 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>>  {
>>  	struct page *swapcache;
>>  	spinlock_t *ptl;
>> -	pte_t *pte;
>> +	pte_t *pte, new_pte;
>>  	int ret = 1;
>>  
>>  	swapcache = page;
>> @@ -1832,8 +1832,14 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>>  		page_add_new_anon_rmap(page, vma, addr);
>>  		lru_cache_add_inactive_or_unevictable(page, vma);
>>  	}
>> -	set_pte_at(vma->vm_mm, addr, pte,
>> -		   pte_mkold(mk_pte(page, vma->vm_page_prot)));
>> +	new_pte = pte_mkold(mk_pte(page, vma->vm_page_prot));
>> +	if (pte_swp_soft_dirty(*pte))
>> +		new_pte = pte_mksoft_dirty(new_pte);
>> +	if (pte_swp_uffd_wp(*pte)) {
>> +		new_pte = pte_mkuffd_wp(new_pte);
>> +		new_pte = pte_wrprotect(new_pte);
> 
> The wrprotect shouldn't be necessary, we don't do a pte_mkwrite(). Note
> that in do_swap_page() we might have done a
> maybe_mkwrite(pte_mkdirty(pte)), which is why the pte_wrprotect() is
> required there.

You're so smart. I happened to be referring to the code in do_swap_page. ;)
Now I see why pte_wrprotect() is only required there. Will remove it in the
next verison when there is enough feedback. Many thanks!

>