Re: [PATCH 02/10] mm/ksm: skip subpages of compound pages

[David Hildenbrand <david@redhat.com>]

On 05.06.24 05:58, Alex Shi wrote:
> 
> 
> On 6/4/24 9:14 PM, David Hildenbrand wrote:
>> On 04.06.24 15:10, Alex Shi wrote:
>>>
>>>
>>> On 6/4/24 6:43 PM, David Hildenbrand wrote:
>>>>>>>
>>>>>>> @@ -2680,7 +2685,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>>>>>>>                  }
>>>>>>>      next_page:
>>>>>>>                  put_page(*page);
>>>>>>> -            ksm_scan.address += PAGE_SIZE;
>>>>>>> +            ksm_scan.address += nr * PAGE_SIZE;
>>>>>>>                  cond_resched();
>>>>>>>              }
>>>>>>>          }
>>>>>>
>>>>>> You might be jumping over pages that don't belong to that folio. What you would actually want to do is somehow use folio_pte_batch() to really know the PTEs point at the same folio, so you can skip them. But that's not that easy when using follow_page() ...
>>>>>>
>>>>>> So I suggest dropping this change for now.
>>>>>>
>>>>>
>>>>> Hi David,
>>>>>
>>>>> Forgive my stupidity, where I jump over normal page that not to belong to the folio?
>>>>
>>>> IIUC, you assume that the folio is fully mapped by all PTEs that could span it, and that follow_page() would give you the head page, correct?
>>>>
>>>> As a simple example, assume only a single page of a large folio is still mapped, which could be any tail page. You couldn't jump over any PTEs.
>>>>
>>>> Or am I missing something?
>>>
>>> Uh, thanks for explanations. for what's we concerned, the following code could take care of the FULL or ERR pages. And it still keep the step of single page.
>>>                           page = follow_page(vma, ksm_scan.address, FOLL_GET);
>>>                           if (IS_ERR_OR_NULL(page)) {
>>>                                   ksm_scan.address += PAGE_SIZE;
>>>                                   cond_resched();
>>>                                   continue;
>>>                           }
>>> And after the above code, step folio_nr_pages on address should be safe, isn't it?
>>
>> Not sure if I follow. Let me try explaining once again:
>>
>> Assume a PTE maps some tail page of the large anonymous folio. The other PTEs around it map some other anonymous folios, not pages of that large anonymous folio.
> 
> 
> Sorry, David,
> 
> Do you meaning there are 2 folio pages, in a same vma, in their address, 'ksm_scan.address', would be overlapped in a folio size space?
> If so, that do out of my expect. I do have no idea of this thing. Could you give me more hints of this problem or how things work on it in current kernel?

We do fully support partially mapping of THPs/large folios. That means, 
you could map a single page of a large pagecache folio and the other 
PTEs could map anonymous folios (due to COW).

Simply because follow_page() returned a page of a large folio doesn't 
generally say that the PTEs around it map the same large folio.

-- 
Cheers,

David / dhildenb