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

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Message-ID: <2fd11f54-8c0c-401d-8635-e54ebf7facc2@bytedance.com>
Date: Wed, 13 Nov 2024 10:40:23 +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 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:

diff --git a/mm/memory.c b/mm/memory.c
index 002aa4f454fa0..2ccdcf37b7a46 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1666,9 +1666,6 @@ 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;
-
         if (pte_present(ptent))
                 return zap_present_ptes(tlb, vma, pte, ptent, max_nr,
                                         addr, details, rss, force_flush,
@@ -1704,11 +1701,15 @@ static unsigned long zap_pte_range(struct 
mmu_gather *tlb,
                 if (need_resched())
                         break;

-               nr = do_zap_pte_range(tlb, vma, pte, addr, end, details, 
rss,
-                                     &force_flush, &force_break);
-               if (unlikely(force_break)) {
-                       addr += nr * PAGE_SIZE;
-                       break;
+               if (pte_none(ptep_get(pte))) {
+                       nr = 1;
+               } else {
+                       nr = do_zap_pte_range(tlb, vma, pte, addr, end, 
details,
+                                             rss, &force_flush, 
&force_break);
+                       if (unlikely(force_break)) {
+                               addr += nr * PAGE_SIZE;
+                               break;
+                       }
                 }
         } while (pte += nr, addr += PAGE_SIZE * nr, addr != end);

This avoids repeated checks for pte_none() later. Both are fine for
me, will change to this in the next version.

> 
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks!

>