Re: [PATCH 3/5] perf/core: Fix arch_perf_get_page_size()

[Liang, Kan]

On 11/13/2020 6:19 AM, Peter Zijlstra wrote:
The (new) page-table walker in arch_perf_get_page_size() is broken in
various ways. Specifically while it is used in a locless manner, it
doesn't depend on CONFIG_HAVE_FAST_GUP nor uses the proper _lockless
offset methods, nor is careful to only read each entry only once.

Also the hugetlb support is broken due to calling pte_page() without
first checking pte_special().

Rewrite the whole thing to be a proper lockless page-table walker and
employ the new pXX_leaf_size() pgtable functions to determine the TLB
size without looking at the page-frames.

Fixes: 51b646b2d9f8 ("perf,mm: Handle non-page-table-aligned hugetlbfs")
Fixes: 8d97e71811aa ("perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE")

The issue 
(https://lkml.kernel.org/r/8e88ba79-7c40-ea32-a7ed-bdc4fc04b...@linux.intel.com) 
has been fixed by this patch set.

Tested-by: Kan Liang <kan.li...@linux.intel.com>


Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
  arch/arm64/include/asm/pgtable.h    |    3 +
  arch/sparc/include/asm/pgtable_64.h |   13 ++++
  arch/sparc/mm/hugetlbpage.c         |   19 ++++--
  include/linux/pgtable.h             |   16 +++++
  kernel/events/core.c                |  102 
+++++++++++++-----------------------
  5 files changed, 82 insertions(+), 71 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7001,90 +7001,62 @@ static u64 perf_virt_to_phys(u64 virt)
        return phys_addr;
  }
  
-#ifdef CONFIG_MMU
-
  /*
- * Return the MMU page size of a given virtual address.
- *
- * This generic implementation handles page-table aligned huge pages, as well
- * as non-page-table aligned hugetlbfs compound pages.
- *
- * If an architecture supports and uses non-page-table aligned pages in their
- * kernel mapping it will need to provide it's own implementation of this
- * function.
+ * Return the MMU/TLB page size of a given virtual address.
   */
-__weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr)
+static u64 perf_get_tlb_page_size(struct mm_struct *mm, unsigned long addr)
  {
-       struct page *page;
-       pgd_t *pgd;
-       p4d_t *p4d;
-       pud_t *pud;
-       pmd_t *pmd;
-       pte_t *pte;
+       u64 size = 0;
  
-	pgd = pgd_offset(mm, addr);
-       if (pgd_none(*pgd))
-               return 0;
+#ifdef CONFIG_HAVE_FAST_GUP
+       pgd_t *pgdp, pgd;
+       p4d_t *p4dp, p4d;
+       pud_t *pudp, pud;
+       pmd_t *pmdp, pmd;
+       pte_t *ptep, pte;
  
-	p4d = p4d_offset(pgd, addr);
-       if (!p4d_present(*p4d))
+       pgdp = pgd_offset(mm, addr);
+       pgd = READ_ONCE(*pgdp);
+       if (pgd_none(pgd))
                return 0;
  
-	if (p4d_leaf(*p4d))
-               return 1ULL << P4D_SHIFT;
+       if (pgd_leaf(pgd))
+               return pgd_leaf_size(pgd);
  
-	pud = pud_offset(p4d, addr);
-       if (!pud_present(*pud))
+       p4dp = p4d_offset_lockless(pgdp, pgd, addr);
+       p4d = READ_ONCE(*p4dp);
+       if (!p4d_present(p4d))
                return 0;
  
-	if (pud_leaf(*pud)) {
-#ifdef pud_page
-               page = pud_page(*pud);
-               if (PageHuge(page))
-                       return page_size(compound_head(page));
-#endif
-               return 1ULL << PUD_SHIFT;
-       }
+       if (p4d_leaf(p4d))
+               return p4d_leaf_size(p4d);
  
-	pmd = pmd_offset(pud, addr);
-       if (!pmd_present(*pmd))
+       pudp = pud_offset_lockless(p4dp, p4d, addr);
+       pud = READ_ONCE(*pudp);
+       if (!pud_present(pud))
                return 0;
  
-	if (pmd_leaf(*pmd)) {
-#ifdef pmd_page
-               page = pmd_page(*pmd);
-               if (PageHuge(page))
-                       return page_size(compound_head(page));
-#endif
-               return 1ULL << PMD_SHIFT;
-       }
+       if (pud_leaf(pud))
+               return pud_leaf_size(pud);
  
-	pte = pte_offset_map(pmd, addr);
-       if (!pte_present(*pte)) {
-               pte_unmap(pte);
+       pmdp = pmd_offset_lockless(pudp, pud, addr);
+       pmd = READ_ONCE(*pmdp);
+       if (!pmd_present(pmd))
                return 0;
-       }
  
-	page = pte_page(*pte);
-       if (PageHuge(page)) {
-               u64 size = page_size(compound_head(page));
-               pte_unmap(pte);
-               return size;
-       }
-
-       pte_unmap(pte);
-       return PAGE_SIZE;
-}
+       if (pmd_leaf(pmd))
+               return pmd_leaf_size(pmd);
  
-#else
+       ptep = pte_offset_map(&pmd, addr);
+       pte = ptep_get_lockless(ptep);
+       if (pte_present(pte))
+               size = pte_leaf_size(pte);
+       pte_unmap(ptep);
+#endif /* CONFIG_HAVE_FAST_GUP */
  
-static u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr)
-{
-       return 0;
+       return size;
  }
  
-#endif
-
  static u64 perf_get_page_size(unsigned long addr)
  {
        struct mm_struct *mm;
@@ -7109,7 +7081,7 @@ static u64 perf_get_page_size(unsigned l
                mm = &init_mm;
        }
  
-	size = arch_perf_get_page_size(mm, addr);
+       size = perf_get_tlb_page_size(mm, addr);
  
  	local_irq_restore(flags);