Re: [PATCH] mm/rmap: Fix potential pte_unmap on an not mapped pte

[Miaohe Lin]

Hi:
On 2021/1/28 8:09, Andrew Morton wrote:
> On Wed, 27 Jan 2021 04:33:49 -0500 Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> 
>> For PMD-mapped page (usually THP), pvmw->pte is NULL. For PTE-mapped THP,
>> pvmw->pte is mapped. But for HugeTLB pages, pvmw->pte is not mapped and set
>> to the relevant page table entry. So in page_vma_mapped_walk_done(), we may
>> do pte_unmap() for HugeTLB pte which is not mapped. Fix this by checking
>> pvmw->page against PageHuge before trying to do pte_unmap().
>>
> 
> What are the runtime consequences of this?  Is there a workload which
> is known to trigger it?
> 

Not yet. This should not be backported. My bad. Sorry about it.

> IOW, how do we justify a -stable backport of this fix?
> >>
>> --- a/include/linux/rmap.h
>> +++ b/include/linux/rmap.h
>> @@ -213,7 +213,8 @@ struct page_vma_mapped_walk {
>>  
>>  static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
>>  {
>> -	if (pvmw->pte)
>> +	/* HugeTLB pte is set to the relevant page table entry without pte_mapped. */
>> +	if (pvmw->pte && !PageHuge(pvmw->page))
>>  		pte_unmap(pvmw->pte);
>>  	if (pvmw->ptl)
>>  		spin_unlock(pvmw->ptl);
>> -- 
>> 2.19.1
> .
>