Re: [PATCH 1/2] mm/numa: no task_numa_fault() call if page table is changed

[David Hildenbrand <david@redhat.com>]

On 08.08.24 05:19, Baolin Wang wrote:
> 
> 
> On 2024/8/8 02:47, Zi Yan wrote:
>> When handling a numa page fault, task_numa_fault() should be called by a
>> process that restores the page table of the faulted folio to avoid
>> duplicated stats counting. Commit b99a342d4f11 ("NUMA balancing: reduce
>> TLB flush via delaying mapping on hint page fault") restructured
>> do_numa_page() and do_huge_pmd_numa_page() and did not avoid
>> task_numa_fault() call in the second page table check after a numa
>> migration failure. Fix it by making all !pte_same()/!pmd_same() return
>> immediately.
>>
>> This issue can cause task_numa_fault() being called more than necessary
>> and lead to unexpected numa balancing results (It is hard to tell whether
>> the issue will cause positive or negative performance impact due to
>> duplicated numa fault counting).
>>
>> Reported-by: "Huang, Ying" <ying.huang@intel.com>
>> Closes: https://lore.kernel.org/linux-mm/87zfqfw0yw.fsf@yhuang6-desk2.ccr.corp.intel.com/
>> Fixes: b99a342d4f11 ("NUMA balancing: reduce TLB flush via delaying mapping on hint page fault")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
> 
> The fix looks reasonable to me. Feel free to add:
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> (Nit: These goto labels are a bit confusing and might need some cleanup
> in the future.)

Agreed, maybe we should simply handle that right away and replace the "goto out;" users by "return 0;".

Then, just copy the 3 LOC.

For mm/memory.c that would be:

diff --git a/mm/memory.c b/mm/memory.c
index 67496dc5064f..410ba50ca746 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5461,7 +5461,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
  
         if (unlikely(!pte_same(old_pte, vmf->orig_pte))) {
                 pte_unmap_unlock(vmf->pte, vmf->ptl);
-               goto out;
+               return 0;
         }
  
         pte = pte_modify(old_pte, vma->vm_page_prot);
@@ -5528,15 +5528,14 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
                 vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
                                                vmf->address, &vmf->ptl);
                 if (unlikely(!vmf->pte))
-                       goto out;
+                       return 0;
                 if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) {
                         pte_unmap_unlock(vmf->pte, vmf->ptl);
-                       goto out;
+                       return 0;
                 }
                 goto out_map;
         }
  
-out:
         if (nid != NUMA_NO_NODE)
                 task_numa_fault(last_cpupid, nid, nr_pages, flags);
         return 0;
@@ -5552,7 +5551,9 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
                 numa_rebuild_single_mapping(vmf, vma, vmf->address, vmf->pte,
                                             writable);
         pte_unmap_unlock(vmf->pte, vmf->ptl);
-       goto out;
+       if (nid != NUMA_NO_NODE)
+               task_numa_fault(last_cpupid, nid, nr_pages, flags);
+       return 0;
  }


-- 
Cheers,

David / dhildenb