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

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Message-ID: <617a063e-bd84-4da5-acf4-6ff516512055@bytedance.com>
Date: Thu, 14 Nov 2024 17:20:04 +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 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()? This
will cause these none ptes to be checked again in count_pte_none() when
PTE_MARKER_UFFD_WP is enabled. Of course, maybe we can count none ptes
in do_zap_pte_range() and pass it through function parameters like 
force_break.

> 
>>           if (pte_present(ptent)) {
>>               nr = zap_present_ptes(tlb, vma, pte, ptent, max_nr,
> 
>