Re: [PATCH v2 3/7] mm: introduce do_zap_pte_range()

[Qi Zheng <zhengqi.arch@bytedance.com>]

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@bytedance.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:

diff --git a/mm/memory.c b/mm/memory.c
index bd9ebe0f4471f..2367a1c19edd6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1657,6 +1657,36 @@ 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)) {
+               int nr = 1;
+
+               for (; nr < max_nr; nr++) {
+                       ptent = ptep_get(pte + nr);
+                       if (!pte_none(ptent))
+                               break;
+               }
+
+               return nr;
+       }
+
+       if (pte_present(ptent))
+               return zap_present_ptes(tlb, vma, pte, ptent, max_nr,
+                                       addr, details, rss, force_flush,
+                                       force_break);
+
+       return zap_nonpresent_ptes(tlb, vma, pte, ptent, max_nr, addr,
+                                        details, rss);
+}
+
  static unsigned long zap_pte_range(struct mmu_gather *tlb,
                                 struct vm_area_struct *vma, pmd_t *pmd,
                                 unsigned long addr, unsigned long end,
@@ -1679,28 +1709,14 @@ static unsigned long zap_pte_range(struct 
mmu_gather *tlb,
         flush_tlb_batched_pending(mm);
         arch_enter_lazy_mmu_mode();
         do {
-               pte_t ptent = ptep_get(pte);
-               int max_nr;
-
-               nr = 1;
-               if (pte_none(ptent))
-                       continue;
-
                 if (need_resched())
                         break;

-               max_nr = (end - addr) / PAGE_SIZE;
-               if (pte_present(ptent)) {
-                       nr = zap_present_ptes(tlb, vma, pte, ptent, max_nr,
-                                             addr, details, rss, 
&force_flush,
-                                             &force_break);
-                       if (unlikely(force_break)) {
-                               addr += nr * PAGE_SIZE;
-                               break;
-                       }
-               } else {
-                       nr = zap_nonpresent_ptes(tlb, vma, pte, ptent, 
max_nr,
-                                                addr, details, rss);
+               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);

> 
> 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.
>