perf/core: Fix arch_perf_get_page_size()

The (new) page-table walker in arch_perf_get_page_size() is broken in
various ways. Specifically while it is used in a lockless 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
pagetable size without looking at the page-frames.

Fixes: 51b646b2 ("perf,mm: Handle non-page-table-aligned hugetlbfs")
Fixes: 8d97e718 ("perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Link: https://lkml.kernel.org/r/20201126124207.GM3040@hirez.programming.kicks-ass.net

kernel/events/core.c

@@ -52,6 +52,7 @@
 #include <linux/mount.h>
 #include <linux/min_heap.h>
 #include <linux/highmem.h>
+#include <linux/pgtable.h>
 
 #include "internal.h"
 
@@ -7001,90 +7002,62 @@ 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 pagetable size of a given virtual address.
  */
-__weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr)
+static u64 perf_get_pgtable_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;
-	}
+	if (pmd_leaf(pmd))
+		return pmd_leaf_size(pmd);
 
-	pte_unmap(pte);
-	return PAGE_SIZE;
-}
+	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 */
 
-#else
-
-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 +7082,7 @@ static u64 perf_get_page_size(unsigned long addr)
 		mm = &init_mm;
 	}
 
-	size = arch_perf_get_page_size(mm, addr);
+	size = perf_get_pgtable_size(mm, addr);
 
 	local_irq_restore(flags);