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

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Message-ID: <fa3fc933-cd51-4be5-944e-250da9289eda@redhat.com>
Date: Thu, 14 Nov 2024 13:32:21 +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 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.

Can't you simply return the number of pte_none that you skipped here using another
input variable, if really required?

-- 
Cheers,

David / dhildenb