Re: [RFC PATCH v2 1/7] mm: pgtable: make pte_offset_map_nolock() return pmdval

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi David,

On 2024/8/6 22:16, David Hildenbrand wrote:
> On 06.08.24 04:40, Qi Zheng wrote:
>> Hi David,
>>
>> On 2024/8/5 22:43, David Hildenbrand wrote:
>>> On 05.08.24 14:55, Qi Zheng wrote:
>>>> Make pte_offset_map_nolock() return pmdval so that we can recheck the
>>>> *pmd once the lock is taken. This is a preparation for freeing empty
>>>> PTE pages, no functional changes are expected.
>>>
>>> Skimming the patches, only patch #4 updates one of the callsites
>>> (collapse_pte_mapped_thp).
>>
>> In addition, retract_page_tables() and reclaim_pgtables_pmd_entry()
>> also used the pmdval returned by pte_offset_map_nolock().
> 
> Right, and I am questioning if only touching these two is sufficient, 
> and how we can make it clearer when someone actually has to recheck the 
> PMD.
> 
>>
>>>
>>> Wouldn't we have to recheck if the PMD val changed in more cases after
>>> taking the PTL?
>>>
>>> If not, would it make sense to have a separate function that returns the
>>> pmdval and we won't have to update each and every callsite?
>>
>> pte_offset_map_nolock() had already obtained the pmdval previously, just
>> hadn't returned it. And updating those callsite is simple, so I think
>> there may not be a need to add a separate function.
> 
> Let me ask this way: why is retract_page_tables() and 
> reclaim_pgtables_pmd_entry() different to the other ones, and how would 
> someone using pte_offset_map_nolock() know what's to do here?

If we acuqire the PTL (PTE or PMD lock) after calling
pte_offset_map_nolock(), it means we may be modifying the corresponding
pte or pmd entry. In that case, we need to perform a pmd_same() check
after holding the PTL, just like in pte_offset_map_lock(), to prevent
the possibility of the PTE page being reclaimed at that time.

If we call pte_offset_map_nolock() and do not need to acquire the PTL
afterwards, it means we are only reading the PTE page. In this case, the
rcu_read_lock() in pte_offset_map_nolock() will ensure that the PTE page
cannot be reclaimed.

> 
> IIUC, we must check the PMDVAL after taking the PTL in case
> 
> (a) we want to modify the page table to turn pte_none() entries to
>      !pte_none(). Because it could be that the page table was removed and
>      now is all pte_none()
> 
> (b) we want to remove the page table ourselves and want to check if it
>      has already been removed.
> 
> Is that it?

Yes.

> 
> So my thinking is if another function variant can make that clearer.

OK, how about naming it pte_offset_map_before_lock?

Thanks,
Qi

>