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

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Message-ID: <2c52e1a0-8431-47ca-9a27-153435805963@bytedance.com>
Date: Thu, 14 Nov 2024 12:12:14 +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 2024/11/14 11:09, Qi Zheng wrote:
> 
> 
> 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;
>> }
> 
> I tend to hand over the pte/addr increments here to the loop
> outside do_zap_pte_range(), like this:
> 

Finally, I choose to introduce skip_none_ptes() to do this:

diff --git a/mm/memory.c b/mm/memory.c
index bd9ebe0f4471f..24633d0e1445a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1657,6 +1657,28 @@ static inline int zap_nonpresent_ptes(struct 
mmu_gather *tlb,
         return nr;
  }

+static inline int skip_none_ptes(pte_t *pte, unsigned long addr,
+                                unsigned long end)
+{
+       pte_t ptent = ptep_get(pte);
+       int max_nr;
+       int nr;
+
+       if (!pte_none(ptent))
+               return 0;
+
+       max_nr = (end - addr) / PAGE_SIZE;
+       nr = 1;
+
+       for (; nr < max_nr; nr++) {
+               ptent = ptep_get(pte + nr);
+               if (!pte_none(ptent))
+                       break;
+       }
+
+       return nr;
+}
+
  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;
                 if (pte_present(ptent)) {
                         nr = zap_present_ptes(tlb, vma, pte, ptent, max_nr,

Thanks!