Re: [PATCH v3 4/9] mm: introduce skip_none_ptes()

From: David Hildenbrand
Date: Thu Nov 14 2024 - 07:32:37 EST


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