Re: [PATCH] mm/mremap: Fix move_normal_pmd/retract_page_tables race

[David Hildenbrand <david@redhat.com>]

On 08.10.24 05:53, Qi Zheng wrote:
> Hi Jann,
> 
> On 2024/10/8 05:42, Jann Horn wrote:
> 
> [...]
> 
>>
>> diff --git a/mm/mremap.c b/mm/mremap.c
>> index 24712f8dbb6b..dda09e957a5d 100644
>> --- a/mm/mremap.c
>> +++ b/mm/mremap.c
>> @@ -238,6 +238,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>>    {
>>    	spinlock_t *old_ptl, *new_ptl;
>>    	struct mm_struct *mm = vma->vm_mm;
>> +	bool res = false;
>>    	pmd_t pmd;
>>    
>>    	if (!arch_supports_page_table_move())
>> @@ -277,19 +278,25 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>>    	if (new_ptl != old_ptl)
>>    		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
>>    
>> -	/* Clear the pmd */
>>    	pmd = *old_pmd;
>> +
>> +	/* Racing with collapse? */
>> +	if (unlikely(!pmd_present(pmd) || pmd_leaf(pmd)))
> 
> Since we already hold the exclusive mmap lock, after a racing
> with collapse occurs, the pmd entry cannot be refilled with
> new content by page fault. So maybe we only need to recheck
> pmd_none(pmd) here?

My thinking was that it is cheap and more future proof to check that we 
really still have a page table here. For example, what if collapse code 
is ever changed to replace the page table by the collapsed PMD?

So unless there is a good reason not to have this check here, I would 
keep it like that.

Thanks!

-- 
Cheers,

David / dhildenb