linux-kernel - Re: [PATCH v1 1/2] mm/userfaultfd: fix uffd-wp handling for THP migration entries

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Date:   Wed, 5 Apr 2023 17:51:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v1 1/2] mm/userfaultfd: fix uffd-wp handling for THP
 migration entries

On 05.04.23 17:43, Peter Xu wrote:
> On Wed, Apr 05, 2023 at 05:17:31PM +0200, David Hildenbrand wrote:
>> On 05.04.23 17:12, Peter Xu wrote:
>>> On Wed, Apr 05, 2023 at 04:25:34PM +0200, David Hildenbrand wrote:
>>>> Looks like what we fixed for hugetlb in commit 44f86392bdd1 ("mm/hugetlb:
>>>> fix uffd-wp handling for migration entries in hugetlb_change_protection()")
>>>> similarly applies to THP.
>>>>
>>>> Setting/clearing uffd-wp on THP migration entries is not implemented
>>>> properly. Further, while removing migration PMDs considers the uffd-wp
>>>> bit, inserting migration PMDs does not consider the uffd-wp bit.
>>>>
>>>> We have to set/clear independently of the migration entry type in
>>>> change_huge_pmd() and properly copy the uffd-wp bit in
>>>> set_pmd_migration_entry().
>>>>
>>>> Verified using a simple reproducer that triggers migration of a THP, that
>>>> the set_pmd_migration_entry() no longer loses the uffd-wp bit.
>>>>
>>>> Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
>>>> Cc: stable@...r.kernel.org
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>
>>> Reviewed-by: Peter Xu <peterx@...hat.com>
>>>
>>> Thanks, one trivial nitpick:
>>>
>>>> ---
>>>>    mm/huge_memory.c | 14 ++++++++++++--
>>>>    1 file changed, 12 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 032fb0ef9cd1..bdda4f426d58 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -1838,10 +1838,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>>>    	if (is_swap_pmd(*pmd)) {
>>>>    		swp_entry_t entry = pmd_to_swp_entry(*pmd);
>>>>    		struct page *page = pfn_swap_entry_to_page(entry);
>>>> +		pmd_t newpmd;
>>>>    		VM_BUG_ON(!is_pmd_migration_entry(*pmd));
>>>>    		if (is_writable_migration_entry(entry)) {
>>>> -			pmd_t newpmd;
>>>>    			/*
>>>>    			 * A protection check is difficult so
>>>>    			 * just be safe and disable write
>>>> @@ -1855,8 +1855,16 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>>>    				newpmd = pmd_swp_mksoft_dirty(newpmd);
>>>>    			if (pmd_swp_uffd_wp(*pmd))
>>>>    				newpmd = pmd_swp_mkuffd_wp(newpmd);
>>>> -			set_pmd_at(mm, addr, pmd, newpmd);
>>>> +		} else {
>>>> +			newpmd = *pmd;
>>>>    		}
>>>> +
>>>> +		if (uffd_wp)
>>>> +			newpmd = pmd_swp_mkuffd_wp(newpmd);
>>>> +		else if (uffd_wp_resolve)
>>>> +			newpmd = pmd_swp_clear_uffd_wp(newpmd);
>>>> +		if (!pmd_same(*pmd, newpmd))
>>>> +			set_pmd_at(mm, addr, pmd, newpmd);
>>>>    		goto unlock;
>>>>    	}
>>>>    #endif
>>>> @@ -3251,6 +3259,8 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
>>>>    	pmdswp = swp_entry_to_pmd(entry);
>>>>    	if (pmd_soft_dirty(pmdval))
>>>>    		pmdswp = pmd_swp_mksoft_dirty(pmdswp);
>>>> +	if (pmd_swp_uffd_wp(*pvmw->pmd))
>>>> +		pmdswp = pmd_swp_mkuffd_wp(pmdswp);
>>>
>>> I think it's fine to use *pmd, but maybe still better to use pmdval?  I
>>> worry pmdp_invalidate()) can be something else in the future that may
>>> affect the bit.
>>
>> Wondering how I ended up with that, I realized that it's actually
>> wrong and might have worked by chance for my reproducer on x86.
>>
>> That should make it work:
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index f977c965fdad..fffc953fa6ea 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3257,7 +3257,7 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
>>          pmdswp = swp_entry_to_pmd(entry);
>>          if (pmd_soft_dirty(pmdval))
>>                  pmdswp = pmd_swp_mksoft_dirty(pmdswp);
>> -       if (pmd_swp_uffd_wp(*pvmw->pmd))
>> +       if (pmd_uffd_wp(pmdval))
>>                  pmdswp = pmd_swp_mkuffd_wp(pmdswp);
>>          set_pmd_at(mm, address, pvmw->pmd, pmdswp);
>>          page_remove_rmap(page, vma, true);
> 
> I guess pmd_swp_uffd_wp() just reads the _USER bit 2 which is also set for
> a present pte, but then it sets swp uffd-wp always even if it was not set.
> 

Yes. I modified the reproducer to migrate without uffd-wp first and we 
suddenly gain a uffd-wp bit.

> Yes the change must be squashed in to be correct, with that, my R-b keeps.

Thanks, I will resend later.

-- 
Thanks,

David / dhildenb