linux-kernel - Re: [PATCH v2 3/7] mm: introduce do_zap_pte

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Message-ID: <303bff80-b535-48a2-bc38-d305cc3fc5c6@bytedance.com>
Date: Wed, 13 Nov 2024 20:19:57 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: jannh@...gle.com, hughd@...gle.com, willy@...radead.org, mgorman@...e.de,
 muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
 zokeefe@...gle.com, rientjes@...gle.com, peterx@...hat.com,
 catalin.marinas@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 x86@...nel.org
Subject: Re: [PATCH v2 3/7] mm: introduce do_zap_pte_range()



On 2024/11/13 19:43, David Hildenbrand wrote:
> On 13.11.24 03:40, Qi Zheng wrote:
>>
>>
>> On 2024/11/13 01:00, David Hildenbrand wrote:
>>> On 31.10.24 09:13, Qi Zheng wrote:
>>>> This commit introduces do_zap_pte_range() to actually zap the PTEs, 
>>>> which
>>>> will help improve code readability and facilitate secondary checking of
>>>> the processed PTEs in the future.
>>>>
>>>> No functional change.
>>>>
>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>>> ---
>>>>    mm/memory.c | 45 ++++++++++++++++++++++++++-------------------
>>>>    1 file changed, 26 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index bd9ebe0f4471f..c1150e62dd073 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -1657,6 +1657,27 @@ static inline int zap_nonpresent_ptes(struct
>>>> mmu_gather *tlb,
>>>>        return nr;
>>>>    }
>>>> +static inline int do_zap_pte_range(struct mmu_gather *tlb,
>>>> +                   struct vm_area_struct *vma, pte_t *pte,
>>>> +                   unsigned long addr, unsigned long end,
>>>> +                   struct zap_details *details, int *rss,
>>>> +                   bool *force_flush, bool *force_break)
>>>> +{
>>>> +    pte_t ptent = ptep_get(pte);
>>>> +    int max_nr = (end - addr) / PAGE_SIZE;
>>>> +
>>>> +    if (pte_none(ptent))
>>>> +        return 1;
>>>
>>> Maybe we should just skip all applicable pte_none() here directly.
>>
>> Do you mean we should keep pte_none() case in zap_pte_range()? Like
>> below:
>>
> 
> No rather an addon patch that will simply skip over all
> consecutive pte_none, like:
> 
> if (pte_none(ptent)) {
>      int nr;
> 
>      for (nr = 1; nr < max_nr; nr++) {
>          ptent = ptep_get(pte + nr);
>          if (pte_none(ptent))
>              continue;
>      }
> 
>      max_nr -= nr;
>      if (!max_nr)
>          return nr;
>      addr += nr * PAGE_SIZE;
>      pte += nr;
> }
> 
> Assuming that it's likely more common to have larger pte_none() holes 
> that single ones, optimizing out the 
> need_resched()+force_break+incremental pte/addr increments etc.

Ah, got it. And I agree with you, will change to it in the next version.

Thanks!

>