Re: [PATCH 13/16] debug_vm_pgtable/pmd_clear: Don't use pmd/pud_clear on pte entries

On 8/13/20 10:57 AM, Anshuman Khandual wrote:

On 08/12/2020 12:03 PM, Aneesh Kumar K.V wrote:

quoted

pmd_clear() should not be used to clear pmd level pte entries.

Could you please elaborate on this. The proposed change set does
not match the description here.

pmd_clear is implemented such that we don't use that to clear a huge pte 
entry. We use pmdp_huge_get_and_clear() for that. Hence we have check in 
pmd_clear which add a WARN if we find a _PAGE_PTE set on the entry.

In the test we follow a hugepmd usage with a pmd_clear. We should 
instead at the end of the advanced pmd test use pmdp_huge_get_and_clear().

quoted

Signed-off-by: Aneesh Kumar K.V <redacted>
---
  mm/debug_vm_pgtable.c | 7 ++++---
  1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 061c19bba7f0..529892b9be2f 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c

@@ -191,6 +191,8 @@ static void __init pmd_advanced_tests(struct mm_struct *mm,
  	pmd = READ_ONCE(*pmdp);
  	WARN_ON(pmd_young(pmd));
  
+	/*  Clear the pte entries  */
+	pmdp_huge_get_and_clear(mm, vaddr, pmdp);
  	pgtable = pgtable_trans_huge_withdraw(mm, pmdp);
  }
  

@@ -313,6 +315,8 @@ static void __init pud_advanced_tests(struct mm_struct *mm,
  	pudp_test_and_clear_young(vma, vaddr, pudp);
  	pud = READ_ONCE(*pudp);
  	WARN_ON(pud_young(pud));
+
+	pudp_huge_get_and_clear(mm, vaddr, pudp);
  }
  
  static void __init pud_leaf_tests(unsigned long pfn, pgprot_t prot)

@@ -431,8 +435,6 @@ static void __init pud_populate_tests(struct mm_struct *mm, pud_t *pudp,
  	 * This entry points to next level page table page.
  	 * Hence this must not qualify as pud_bad().
  	 */
-	pmd_clear(pmdp);
-	pud_clear(pudp);

Both entires are cleared before creating a fresh page table entry.
Why that is a problem.

quoted

  	pud_populate(mm, pudp, pmdp);
  	pud = READ_ONCE(*pudp);
  	WARN_ON(pud_bad(pud));

@@ -564,7 +566,6 @@ static void __init pmd_populate_tests(struct mm_struct *mm, pmd_t *pmdp,
  	 * This entry points to next level page table page.
  	 * Hence this must not qualify as pmd_bad().
  	 */
-	pmd_clear(pmdp);

Ditto.

quoted

  	pmd_populate(mm, pmdp, pgtable);
  	pmd = READ_ONCE(*pmdp);
  	WARN_ON(pmd_bad(pmd));