Re: [PATCH v2 28/32] mm/memory: allow pte_offset_map[_lock]() to fail

[Ryan Roberts]

On 09/06/2023 21:11, Hugh Dickins wrote:
> On Fri, 9 Jun 2023, Andrew Morton wrote:
>> On Thu, 8 Jun 2023 18:43:38 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>>
>>> copy_pte_range(): use pte_offset_map_nolock(), and allow for it to fail;
>>> but with a comment on some further assumptions that are being made there.
>>>
>>> zap_pte_range() and zap_pmd_range(): adjust their interaction so that
>>> a pte_offset_map_lock() failure in zap_pte_range() leads to a retry in
>>> zap_pmd_range(); remove call to pmd_none_or_trans_huge_or_clear_bad().
>>>
>>> Allow pte_offset_map_lock() to fail in many functions.  Update comment
>>> on calling pte_alloc() in do_anonymous_page().  Remove redundant calls
>>> to pmd_trans_unstable(), pmd_devmap_trans_unstable(), pmd_none() and
>>> pmd_bad(); but leave pmd_none_or_clear_bad() calls in free_pmd_range()
>>> and copy_pmd_range(), those do simplify the next level down.
>>>
>>> ...
>>>
>>> @@ -3728,11 +3737,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>  			vmf->page = pfn_swap_entry_to_page(entry);
>>>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>>  					vmf->address, &vmf->ptl);
>>> -			if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
>>> -				spin_unlock(vmf->ptl);
>>> -				goto out;
>>> -			}
>>> -
>>> +			if (unlikely(!vmf->pte ||
>>> +				     !pte_same(*vmf->pte, vmf->orig_pte)))
>>> +				goto unlock;
>>>  			/*
>>>  			 * Get a page reference while we know the page can't be
>>>  			 * freed.
>>
>> This hunk falls afoul of
>> https://lkml.kernel.org/r/20230602092949.545577-5-ryan.roberts@xxxxxxx.
>>
>> I did this:
>>
>> @@ -3729,7 +3738,8 @@ vm_fault_t do_swap_page(struct vm_fault
>>  			vmf->page = pfn_swap_entry_to_page(entry);
>>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>  					vmf->address, &vmf->ptl);
>> -			if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte)))
>> +			if (unlikely(!vmf->pte ||
>> +				     !pte_same(*vmf->pte, vmf->orig_pte)))
>>  				goto unlock;
>>  
>>  			/*
> 
> Yes, that's exactly right: thanks, Andrew.

FWIW, I agree.

Thanks,
Ryan


> 
> Hugh