Re: [PATCH v3 04/13] powerpc: assert_pte_locked() use pte_offset_map_nolock()

[Aneesh Kumar K V]

On 7/19/23 10:34 AM, Hugh Dickins wrote:
> On Tue, 18 Jul 2023, Aneesh Kumar K.V wrote:
>> Hugh Dickins <hughd@xxxxxxxxxx> writes:
>>
>>> Instead of pte_lockptr(), use the recently added pte_offset_map_nolock()
>>> in assert_pte_locked().  BUG if pte_offset_map_nolock() fails: this is
>>> stricter than the previous implementation, which skipped when pmd_none()
>>> (with a comment on khugepaged collapse transitions): but wouldn't we want
>>> to know, if an assert_pte_locked() caller can be racing such transitions?
>>>
>>
>> The reason we had that pmd_none check there was to handle khugpaged. In
>> case of khugepaged we do pmdp_collapse_flush and then do a ptep_clear.
>> ppc64 had the assert_pte_locked check inside that ptep_clear.
>>
>> _pmd = pmdp_collapse_flush(vma, address, pmd);
>> ..
>> ptep_clear()
>> -> asset_ptep_locked()
>> ---> pmd_none
>> -----> BUG
>>
>>
>> The problem is how assert_pte_locked() verify whether we are holding
>> ptl. It does that by walking the page table again and in this specific
>> case by the time we call the function we already had cleared pmd .
> 
> Aneesh, please clarify, I've spent hours on this.
> 
> From all your use of past tense ("had"), I thought you were Acking my
> patch; but now, after looking again at v3.11 source and today's,
> I think you are NAKing my patch in its present form.
> 

Sorry for the confusion my reply created. 

> You are pointing out that anon THP's __collapse_huge_page_copy_succeeded()
> uses ptep_clear() at a point after pmdp_collapse_flush() already cleared
> *pmd, so my patch now leads that one use of assert_pte_locked() to BUG.
> Is that your point?
> 

Yes. I haven't tested this yet to verify that it is indeed hitting that BUG.
But a code inspection tells me we will hit that BUG on powerpc because of
the above details.

> I can easily restore that khugepaged comment (which had appeared to me
> out of date at the time, but now looks still relevant) and pmd_none(*pmd)
> check: but please clarify.
> 

That is correct. if we add that pmd_none check back we should be good here.


-aneesh