Re: [PATCH] mm/page_table_check: Fix crash on ZONE_DEVICE

[Pasha Tatashin]

On Wed, Jun 5, 2024 at 5:21 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> Not all pages may apply to pgtable check.  One example is ZONE_DEVICE
> pages: they map PFNs directly, and they don't allocate page_ext at all even
> if there's struct page around.  One may reference devm_memremap_pages().
>
> When both ZONE_DEVICE and page-table-check enabled, then try to map some
> dax memories, one can trigger kernel bug constantly now when the kernel was
> trying to inject some pfn maps on the dax device:
>
>  kernel BUG at mm/page_table_check.c:55!
>
> While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page
> fault resolutions, skip all the checks if page_ext doesn't even exist in
> pgtable checker, which applies to ZONE_DEVICE but maybe more.

Thank you for reporting this bug. A few comments below:

>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  mm/page_table_check.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index 4169576bed72..509c6ef8de40 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -73,6 +73,9 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
>         page = pfn_to_page(pfn);
>         page_ext = page_ext_get(page);
>
> +       if (!page_ext)
> +               return;

I would replace the above with the following, here and in other places:

if (!page_ext) {
  WARN_ONCE(!is_zone_device_page(page),
                          "page_ext is missing for a non-device page\n");
  return;
}

> +
>         BUG_ON(PageSlab(page));
>         anon = PageAnon(page);
>
> @@ -110,6 +113,9 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
>         page = pfn_to_page(pfn);
>         page_ext = page_ext_get(page);
>
> +       if (!page_ext)
> +               return;
> +
>         BUG_ON(PageSlab(page));
>         anon = PageAnon(page);
>
> @@ -140,7 +146,10 @@ void __page_table_check_zero(struct page *page, unsigned int order)
>         BUG_ON(PageSlab(page));
>
>         page_ext = page_ext_get(page);
> -       BUG_ON(!page_ext);
> +
> +       if (!page_ext)
> +               return;
> +
>         for (i = 0; i < (1ul << order); i++) {
>                 struct page_table_check *ptc = get_page_table_check(page_ext);
>
> --
> 2.45.0
>