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

[Alex Shi <seakeel@gmail.com>]

On 6/4/24 4:12 PM, David Hildenbrand wrote:
> On 04.06.24 06:24, alexs@kernel.org wrote:
>> From: "Alex Shi (tencent)" <alexs@kernel.org>
>>
>> When a folio isn't fit for KSM, the subpages are unlikely to be good,
>> So let's skip the rest page checking to save some actions.
>>
>> Signed-off-by: Alex Shi (tencent) <alexs@kernel.org>
>> ---
>>   mm/ksm.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 97e5b41f8c4b..e2fdb9dd98e2 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -2644,6 +2644,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>>           goto no_vmas;
>>         for_each_vma(vmi, vma) {
>> +        int nr = 1;
>> +
>>           if (!(vma->vm_flags & VM_MERGEABLE))
>>               continue;
>>           if (ksm_scan.address < vma->vm_start)
>> @@ -2660,6 +2662,9 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>>                   cond_resched();
>>                   continue;
>>               }
>> +
>> +            VM_WARN_ON(PageTail(*page));
>> +            nr = compound_nr(*page);
>>               if (is_zone_device_page(*page))
>>                   goto next_page;
>>               if (PageAnon(*page)) {
>> @@ -2672,7 +2677,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>>                       if (should_skip_rmap_item(*page, rmap_item))
>>                           goto next_page;
>>   -                    ksm_scan.address += PAGE_SIZE;
>> +                    ksm_scan.address += nr * PAGE_SIZE;
>>                   } else
>>                       put_page(*page);
>>                   mmap_read_unlock(mm);
>> @@ -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?

Thanks
Alex