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

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Message-ID: <20240807184730.1266736-1-ziy@nvidia.com>
Date: Wed,  7 Aug 2024 14:47:29 -0400
From: Zi Yan <ziy@...dia.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	linux-kernel@...r.kernel.org,
	Zi Yan <ziy@...dia.com>,
	stable@...r.kernel.org
Subject: [PATCH 1/2] mm/numa: no task_numa_fault() call if page table is changed

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@...el.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@...r.kernel.org>
Signed-off-by: Zi Yan <ziy@...dia.com>
---
 mm/huge_memory.c | 5 +++--
 mm/memory.c      | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0024266dea0a..a3c018f2b554 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1734,10 +1734,11 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 		goto out_map;
 	}
 
-out:
+count_fault:
 	if (nid != NUMA_NO_NODE)
 		task_numa_fault(last_cpupid, nid, HPAGE_PMD_NR, flags);
 
+out:
 	return 0;
 
 out_map:
@@ -1749,7 +1750,7 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 	set_pmd_at(vma->vm_mm, haddr, vmf->pmd, pmd);
 	update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
 	spin_unlock(vmf->ptl);
-	goto out;
+	goto count_fault;
 }
 
 /*
diff --git a/mm/memory.c b/mm/memory.c
index 67496dc5064f..503d493263df 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5536,9 +5536,10 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
 		goto out_map;
 	}
 
-out:
+count_fault:
 	if (nid != NUMA_NO_NODE)
 		task_numa_fault(last_cpupid, nid, nr_pages, flags);
+out:
 	return 0;
 out_map:
 	/*
@@ -5552,7 +5553,7 @@ 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;
+	goto count_fault;
 }
 
 static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
-- 
2.43.0