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

[Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>]

On Mon, Aug 11, 2025 at 01:26:28PM +0200, 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@xxxxxxxxxx>

This LGTM, great explanations and thanks for the page table level stuff!

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  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";
> +     }
> +}
> +
>  #endif /* !__ASSEMBLY__ */
>
>  #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
> diff --git a/mm/memory.c b/mm/memory.c
> index 626caedce35e0..dc0107354d37b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -491,22 +491,8 @@ static inline void add_mm_rss_vec(struct mm_struct *mm, 
> int *rss)
>                       add_mm_counter(mm, i, rss[i]);
>  }
>
> -/*
> - * This function is called to print an error when a bad pte
> - * is found. For example, we might have a PFN-mapped pte in
> - * a region that doesn't allow it.
> - *
> - * The calling function must still handle the error.
> - */
> -static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
> -                       pte_t pte, struct page *page)
> +static bool is_bad_page_map_ratelimited(void)
>  {
> -     pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> -     p4d_t *p4d = p4d_offset(pgd, addr);
> -     pud_t *pud = pud_offset(p4d, addr);
> -     pmd_t *pmd = pmd_offset(pud, addr);
> -     struct address_space *mapping;
> -     pgoff_t index;
>       static unsigned long resume;
>       static unsigned long nr_shown;
>       static unsigned long nr_unshown;
> @@ -518,7 +504,7 @@ static void print_bad_pte(struct vm_area_struct *vma, 
> unsigned long addr,
>       if (nr_shown == 60) {
>               if (time_before(jiffies, resume)) {
>                       nr_unshown++;
> -                     return;
> +                     return true;
>               }
>               if (nr_unshown) {
>                       pr_alert("BUG: Bad page map: %lu messages suppressed\n",
> @@ -529,15 +515,91 @@ static void print_bad_pte(struct vm_area_struct *vma, 
> unsigned long addr,
>       }
>       if (nr_shown++ == 0)
>               resume = jiffies + 60 * HZ;
> +     return false;
> +}
> +
> +static void __print_bad_page_map_pgtable(struct mm_struct *mm, unsigned long 
> addr)
> +{
> +     unsigned long long pgdv, p4dv, pudv, pmdv;
> +     p4d_t p4d, *p4dp;
> +     pud_t pud, *pudp;
> +     pmd_t pmd, *pmdp;
> +     pgd_t *pgdp;
> +
> +     /*
> +      * Although this looks like a fully lockless pgtable walk, it is not:
> +      * see locking requirements for print_bad_page_map().
> +      */

Thanks

> +     pgdp = pgd_offset(mm, addr);
> +     pgdv = pgd_val(*pgdp);
> +
> +     if (!pgd_present(*pgdp) || pgd_leaf(*pgdp)) {
> +             pr_alert("pgd:%08llx\n", pgdv);
> +             return;
> +     }
> +
> +     p4dp = p4d_offset(pgdp, addr);
> +     p4d = p4dp_get(p4dp);
> +     p4dv = p4d_val(p4d);
> +
> +     if (!p4d_present(p4d) || p4d_leaf(p4d)) {
> +             pr_alert("pgd:%08llx p4d:%08llx\n", pgdv, p4dv);
> +             return;
> +     }
> +
> +     pudp = pud_offset(p4dp, addr);
> +     pud = pudp_get(pudp);
> +     pudv = pud_val(pud);
> +
> +     if (!pud_present(pud) || pud_leaf(pud)) {
> +             pr_alert("pgd:%08llx p4d:%08llx pud:%08llx\n", pgdv, p4dv, 
> pudv);
> +             return;
> +     }
> +
> +     pmdp = pmd_offset(pudp, addr);
> +     pmd = pmdp_get(pmdp);
> +     pmdv = pmd_val(pmd);
> +
> +     /*
> +      * Dumping the PTE would be nice, but it's tricky with CONFIG_HIGHPTE,

Sigh, 32-bit.

> +      * because the table should already be mapped by the caller and
> +      * doing another map would be bad. print_bad_page_map() should
> +      * already take care of printing the PTE.
> +      */
> +     pr_alert("pgd:%08llx p4d:%08llx pud:%08llx pmd:%08llx\n", pgdv,
> +              p4dv, pudv, pmdv);
> +}
> +
> +/*
> + * This function is called to print an error when a bad page table entry 
> (e.g.,
> + * corrupted page table entry) is found. For example, we might have a
> + * PFN-mapped pte in a region that doesn't allow it.
> + *
> + * The calling function must still handle the error.
> + *
> + * This function must be called during a proper page table walk, as it will
> + * re-walk the page table to dump information: the caller MUST prevent page
> + * table teardown (by holding mmap, vma or rmap lock) and MUST hold the leaf
> + * page table lock.
> + */

Thanks this is good!

> +static void print_bad_page_map(struct vm_area_struct *vma,
> +             unsigned long addr, unsigned long long entry, struct page *page,
> +             enum pgtable_level level)
> +{
> +     struct address_space *mapping;
> +     pgoff_t index;
> +
> +     if (is_bad_page_map_ratelimited())
> +             return;
>
>       mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
>       index = linear_page_index(vma, addr);
>
> -     pr_alert("BUG: Bad page map in process %s  pte:%08llx pmd:%08llx\n",
> -              current->comm,
> -              (long long)pte_val(pte), (long long)pmd_val(*pmd));
> +     pr_alert("BUG: Bad page map in process %s  %s:%08llx", current->comm,
> +              pgtable_level_to_str(level), entry);
> +     __print_bad_page_map_pgtable(vma->vm_mm, addr);
>       if (page)
> -             dump_page(page, "bad pte");
> +             dump_page(page, "bad page map");
>       pr_alert("addr:%px vm_flags:%08lx anon_vma:%px mapping:%px index:%lx\n",
>                (void *)addr, vma->vm_flags, vma->anon_vma, mapping, index);
>       pr_alert("file:%pD fault:%ps mmap:%ps mmap_prepare: %ps 
> read_folio:%ps\n",
> @@ -549,6 +611,8 @@ static void print_bad_pte(struct vm_area_struct *vma, 
> unsigned long addr,
>       dump_stack();
>       add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  }
> +#define print_bad_pte(vma, addr, pte, page) \
> +     print_bad_page_map(vma, addr, pte_val(pte), page, PGTABLE_LEVEL_PTE)

This is a nice abstraction.

>
>  /*
>   * vm_normal_page -- This function gets the "struct page" associated with a 
> pte.
> --
> 2.50.1
>