linux-kernel - Re: [PATCH v3 4/9] mm: introduce skip_none

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Message-ID: <d27a75fa-b968-43d3-bbd3-cc607feee495@redhat.com>
Date: Thu, 14 Nov 2024 22:19:00 +0100
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: jannh@...gle.com, hughd@...gle.com, willy@...radead.org,
 muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
 peterx@...hat.com, mgorman@...e.de, catalin.marinas@....com,
 will@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
 peterz@...radead.org, x86@...nel.org, lorenzo.stoakes@...cle.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, zokeefe@...gle.com,
 rientjes@...gle.com
Subject: Re: [PATCH v3 4/9] mm: introduce skip_none_ptes()

On 14.11.24 13:51, Qi Zheng wrote:
> 
> 
> On 2024/11/14 20:32, David Hildenbrand wrote:
>> On 14.11.24 10:20, Qi Zheng wrote:
>>>
>>>
>>> On 2024/11/14 16:04, David Hildenbrand wrote:
>>>>
>>>>>     static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>>>>                     struct vm_area_struct *vma, pmd_t *pmd,
>>>>>                     unsigned long addr, unsigned long end,
>>>>> @@ -1682,13 +1704,17 @@ static unsigned long zap_pte_range(struct
>>>>> mmu_gather *tlb,
>>>>>             pte_t ptent = ptep_get(pte);
>>>>>             int max_nr;
>>>>> -        nr = 1;
>>>>> -        if (pte_none(ptent))
>>>>> -            continue;
>>>>> -
>>>>>             if (need_resched())
>>>>>                 break;
>>>>> +        nr = skip_none_ptes(pte, addr, end);
>>>>> +        if (nr) {
>>>>> +            addr += PAGE_SIZE * nr;
>>>>> +            if (addr == end)
>>>>> +                break;
>>>>> +            pte += nr;
>>>>> +        }
>>>>> +
>>>>>             max_nr = (end - addr) / PAGE_SIZE;
>>>>
>>>> I dislike calculating max_nr twice, once here and once in skip_non_ptes.
>>>>
>>>> Further, you're missing to update ptent here.
>>>
>>> Oh, my bad. However, with [PATCH v3 5/9], there will be no problem, but
>>> there are still two ptep_get() and max_nr calculation.
>>>
>>> If you inline it you can
>>>> avoid another ptep_get().
>>>
>>> Do you mean to inline the skip_none_ptes() into do_zap_pte_range()?
>>
>> Effectively moving this patch after #5, and have it be something like:
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 1949f5e0fece5..4f5d1e4c6688e 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1667,8 +1667,21 @@ static inline int do_zap_pte_range(struct
>> mmu_gather *tlb,
>>           pte_t ptent = ptep_get(pte);
>>           int max_nr = (end - addr) / PAGE_SIZE;
>>
>> -       if (pte_none(ptent))
>> -               return 1;
>> +       /* Skip all consecutive pte_none(). */
>> +       if (pte_none(ptent)) {
>> +               int nr;
>> +
>> +               for (nr = 1; nr < max_nr; nr++) {
>> +                       ptent = ptep_get(pte + nr);
>> +                       if (!pte_none(ptent))
>> +                               break;
>> +               }
>> +               max_nr -= nr;
>> +               if (!max_nr)
>> +                       return nr;
>> +               pte += nr;
>> +               addr += nr * PAGE_SIZE;
>> +       }
>>
>>           if (pte_present(ptent))
>>                   return zap_present_ptes(tlb, vma, pte, ptent, max_nr,
>>
>>
>> In the context of this patch this makes most sense.
>>
>> Regarding "count_pte_none" comment, I assume you talk about patch #7.
> 
> Yes.
> 
>>
>> Can't you simply return the number of pte_none that you skipped here
>> using another
>> input variable, if really required?
> 
> Suppose we add an input variable nr_skip to do_zap_pte_range(), you mean
> to return the above nr to zap_pte_range() through:

Maybe "cur_none_nr" or something similar.

> 
> *nr_skip = nr;
> 
> and then:
> 
> zap_pte_range
> --> nr = do_zap_pte_range(tlb, vma, pte, addr, end, details, &skip_nr,
>    				      rss, &force_flush, &force_break);
>       if (can_reclaim_pt) {
>           none_nr += count_pte_none(pte, nr);
>           none_nr += nr_skip;
>       }
> 
> Right?

Yes. I did not look closely at the patch that adds the counting of 
pte_none though (to digest why it is required :) ).

-- 
Cheers,

David / dhildenb