[PATCH v3 1/3] mm/numa: no task_numa_fault() call if PTE is changed

[Zi Yan <ziy@nvidia.com>]

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 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() 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>
---
 mm/memory.c | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 67496dc5064f..bf791da57cab 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);
@@ -5523,23 +5523,19 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
 	if (!migrate_misplaced_folio(folio, vma, target_nid)) {
 		nid = target_nid;
 		flags |= TNF_MIGRATED;
-	} else {
-		flags |= TNF_MIGRATE_FAIL;
-		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-					       vmf->address, &vmf->ptl);
-		if (unlikely(!vmf->pte))
-			goto out;
-		if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) {
-			pte_unmap_unlock(vmf->pte, vmf->ptl);
-			goto out;
-		}
-		goto out_map;
+		task_numa_fault(last_cpupid, nid, nr_pages, flags);
+		return 0;
 	}
 
-out:
-	if (nid != NUMA_NO_NODE)
-		task_numa_fault(last_cpupid, nid, nr_pages, flags);
-	return 0;
+	flags |= TNF_MIGRATE_FAIL;
+	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
+				       vmf->address, &vmf->ptl);
+	if (unlikely(!vmf->pte))
+		return 0;
+	if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) {
+		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		return 0;
+	}
 out_map:
 	/*
 	 * Make it present again, depending on how arch implements
@@ -5552,7 +5548,10 @@ 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;
 }
 
 static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
-- 
2.43.0