On 26.08.25 07:25, Lorenzo Stoakes wrote:

On Mon, Aug 25, 2025 at 02:31:00PM +0200, David Hildenbrand wrote:

quoted

On 11.08.25 13:26, David Hildenbrand wrote:

quoted

print_bad_pte() looks like something that should actually be a WARN
or similar, but historically it apparently has proven to be useful to
detect corruption of page tables even on production systems -- report
the issue and keep the system running to make it easier to actually detect
what is going wrong (e.g., multiple such messages might shed a light).

As we want to unify vm_normal_page_*() handling for PTE/PMD/PUD, we'll have
to take care of print_bad_pte() as well.

Let's prepare for using print_bad_pte() also for non-PTEs by adjusting the
implementation and renaming the function to print_bad_page_map().
Provide print_bad_pte() as a simple wrapper.

Document the implicit locking requirements for the page table re-walk.

To make the function a bit more readable, factor out the ratelimit check
into is_bad_page_map_ratelimited() and place the printing of page
table content into __print_bad_page_map_pgtable(). We'll now dump
information from each level in a single line, and just stop the table
walk once we hit something that is not a present page table.

The report will now look something like (dumping pgd to pmd values):

[   77.943408] BUG: Bad page map in process XXX  pte:80000001233f5867
[   77.944077] addr:00007fd84bb1c000 vm_flags:08100071 anon_vma: ...
[   77.945186] pgd:10a89f067 p4d:10a89f067 pud:10e5a2067 pmd:105327067

Not using pgdp_get(), because that does not work properly on some arm
configs where pgd_t is an array. Note that we are dumping all levels
even when levels are folded for simplicity.

Signed-off-by: David Hildenbrand <redacted>
---
   include/linux/pgtable.h |  19 ++++++++
   mm/memory.c             | 104 ++++++++++++++++++++++++++++++++--------
   2 files changed, 103 insertions(+), 20 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index bff5c4241bf2e..33c84b38b7ec6 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h

@@ -1966,6 +1966,25 @@ enum pgtable_level {
   	PGTABLE_LEVEL_PGD,
   };
+static inline const char *pgtable_level_to_str(enum pgtable_level level)
+{
+	switch (level) {
+	case PGTABLE_LEVEL_PTE:
+		return "pte";
+	case PGTABLE_LEVEL_PMD:
+		return "pmd";
+	case PGTABLE_LEVEL_PUD:
+		return "pud";
+	case PGTABLE_LEVEL_P4D:
+		return "p4d";
+	case PGTABLE_LEVEL_PGD:
+		return "pgd";
+	default:
+		VM_WARN_ON_ONCE(1);
+		return "unknown";
+	}
+}

One kernel config doesn't like the VM_WARN_ON_ONCE here, and I don't think we
really need it. @Andrew can you squash:

Out of interest do you know why this is happening? xtensa right? Does
xtensa not like CONFIG_DEBUG_VM?

We don't happen to include mmdebug.h in a xtensa configuration.

Briefly thought about using a BUILD_BUG_ON_INVALID(), but decided to 
just drop it completely.

-- 
Cheers

David / dhildenb