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

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Date: Tue, 4 Jun 2024 12:43:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Alex Shi <seakeel@...il.com>, alexs@...nel.org,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, izik.eidus@...ellosystems.com,
 willy@...radead.org, aarcange@...hat.com, chrisw@...s-sol.org,
 hughd@...gle.com
Subject: Re: [PATCH 02/10] mm/ksm: skip subpages of compound pages

On 04.06.24 12:31, Alex Shi wrote:
> 
> 
> On 6/4/24 4:12 PM, David Hildenbrand wrote:
>> On 04.06.24 06:24, alexs@...nel.org wrote:
>>> From: "Alex Shi (tencent)" <alexs@...nel.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@...nel.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?

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?

-- 
Cheers,

David / dhildenb