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

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Message-ID: <b524a568-fa3b-4618-80cc-e8c31ea4eeac@bytedance.com>
Date: Thu, 14 Nov 2024 20:51:27 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.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 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:

*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?

>