Re: [PATCH v3 13/13] mm/debug_vm_pgtable: populate a pte entry before fetching it

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On 9/2/20 9:19 AM, Anshuman Khandual wrote:


On 09/01/2020 03:28 PM, Aneesh Kumar K.V wrote:
On 9/1/20 1:08 PM, Anshuman Khandual wrote:


On 09/01/2020 12:07 PM, Aneesh Kumar K.V wrote:
On 9/1/20 8:55 AM, Anshuman Khandual wrote:


On 08/27/2020 01:34 PM, Aneesh Kumar K.V wrote:
pte_clear_tests operate on an existing pte entry. Make sure that is not a none
pte entry.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
---
    mm/debug_vm_pgtable.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 21329c7d672f..8527ebb75f2c 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c
@@ -546,7 +546,7 @@ static void __init pgd_populate_tests(struct mm_struct *mm, pgd_t *pgdp,
    static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep,
                       unsigned long vaddr)
    {
-    pte_t pte = ptep_get(ptep);
+    pte_t pte =  ptep_get_and_clear(mm, vaddr, ptep);

Seems like ptep_get_and_clear() here just clears the entry in preparation
for a following set_pte_at() which otherwise would have been a problem on
ppc64 as you had pointed out earlier i.e set_pte_at() should not update an
existing valid entry. So the commit message here is bit misleading.


and also fetch the pte value which is used further.


          pr_debug("Validating PTE clear\n");
        pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
@@ -944,7 +944,7 @@ static int __init debug_vm_pgtable(void)
        p4d_t *p4dp, *saved_p4dp;
        pud_t *pudp, *saved_pudp;
        pmd_t *pmdp, *saved_pmdp, pmd;
-    pte_t *ptep;
+    pte_t *ptep, pte;
        pgtable_t saved_ptep;
        pgprot_t prot, protnone;
        phys_addr_t paddr;
@@ -1049,6 +1049,8 @@ static int __init debug_vm_pgtable(void)
         */
          ptep = pte_alloc_map_lock(mm, pmdp, vaddr, &ptl);
+    pte = pfn_pte(pte_aligned, prot);
+    set_pte_at(mm, vaddr, ptep, pte);

Not here, creating and populating an entry must be done in respective
test functions itself. Besides, this seems bit redundant as well. The
test pte_clear_tests() with the above change added, already

- Clears the PTEP entry with ptep_get_and_clear()

and fetch the old value set previously.

In that case, please move above two lines i.e

pte = pfn_pte(pte_aligned, prot);
set_pte_at(mm, vaddr, ptep, pte);

from debug_vm_pgtable() to pte_clear_tests() and update it's arguments
as required.


Frankly, I don't understand what these tests are testing. It all looks like some random clear and set.

The idea here is to have some value with some randomness preferably, in
a given PTEP before attempting to clear the entry, in order to make sure
that pte_clear() is indeed clearing something of non-zero value.


static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep,
                    unsigned long vaddr, unsigned long pfn,
                    pgprot_t prot)
{

     pte_t pte = pfn_pte(pfn, prot);
     set_pte_at(mm, vaddr, ptep, pte);

     pte =  ptep_get_and_clear(mm, vaddr, ptep);

Looking at this again, this preceding pfn_pte() followed by set_pte_at()
is not really required. Its reasonable to start with what ever was there
in the PTEP as a seed value which anyway gets added with RANDOM_ORVALUE.
s/ptep_get/ptep_get_and_clear is sufficient to take care of the powerpc
set_pte_at() constraint.


But the way test is written we had none pte before. That is why I added that set_pte_at to put something there. With none pte the below sequence fails.

	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
	set_pte_at(mm, vaddr, ptep, pte);


because nobody is marking a _PAGE_PTE there.

	pte_t pte = pfn_pte(pfn, prot);

	pr_debug("Validating PTE clear\n");
	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
	set_pte_at(mm, vaddr, ptep, pte);
	barrier();
	pte_clear(mm, vaddr, ptep);
	pte = ptep_get(ptep);
	WARN_ON(!pte_none(pte));

will that work for you?

-aneesh



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