Re: [PATCH v3 08/11] mm/memory: convert print_bad_pte() to print_bad_page_map()

[David Hildenbrand <david@redhat.com>]

On 11.08.25 13:26, David Hildenbrand wrote:
> 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 <david@redhat.com>
> ---
>   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:

 From 0b8f6cdfe2c9d96393e7da1772e82048e096a903 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Mon, 25 Aug 2025 14:25:59 +0200
Subject: [PATCH] fixup: mm/memory: convert print_bad_pte() to
  print_bad_page_map()

Let's just drop the warning, it's highly unlikely that we ever run into
this, and if so, there is serious stuff going wrong elsewhere.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
  include/linux/pgtable.h | 1 -
  1 file changed, 1 deletion(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 9f0329d45b1e1..94249e671a7e8 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1997,7 +1997,6 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level)
  	case PGTABLE_LEVEL_PGD:
  		return "pgd";
  	default:
-		VM_WARN_ON_ONCE(1);
  		return "unknown";
  	}
  }
-- 
2.50.1


-- 
Cheers

David / dhildenb